bison shepherd's pie with rosemary peppercorn brown butter and sour cream & onion potatoes

- entreés, meal prep -

two of my favorite things: cast iron pans and meal prepping lunches. this recipe gives me the both of best worlds. i'll be upfront with you, though. it is not a 30-minute dish. it is not a quick recipe, but it takes time to build the flavors you get out of it in the end. the extra steps of sautéing the veggies separately allow each one to be properly cooked. most of it could be made ahead as well, only requiring you to put the potatoes on top and pop it in the oven the day of.
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INGREDIENTS

1 stick of unsalted butter
​2-3 whole sprigs of fresh rosemary
1 tbsp whole black peppercorns
4-5 russet potatoes, peeled
16 oz baby bella mushrooms, cleaned and diced *
4-5 medium-large carrots, diced * 
2 medium yellow onions, diced * 
2 red bell peppers, diced *
24 oz ground bison (you also could use ground beef)
2 c + 2 tablespoons low/no sodium beef broth, divided
2 1/4 tsp salt, divided, plus some for salting boiling water
2 tbsp low sodium soy sauce, divided
4 tbsp Worcestershire sauce, divided
2/3 cup light sour cream (you can use full-fat, I just used what we had in the fridge)
1/4 tsp dried chives
2 tbsp garlic powder
1 1/2 tsp onion powder
5 tsp onion salt (or 3 tsp onion powder, 1 tsp dried chives, and 1 tsp salt), plus some for topping
2 tsp cornstarch
3 egg yolks
1/2 tsp cracked black pepper
* - should be the same size and a pretty small dice

DIRECTIONS

1. start by placing the peeled potatoes in a large pot and cover in 3/4 cup of cold water. add a few tablespoons of salt. boil on high heat for 30-55 minutes, depending on the shape and size of your pot. you want to be able to easily pierce the potato with a fork and for it to be tender.  once ready, strain them in a colander. 
2. make the rosemary peppercorn brown butter. slice up the stick of butter and add it along with the peppercorns and rosemary to a medium-large pot. place it on the stove top on medium heat, swirling it to coat the rosemary sprigs. once the rosemary starts to get brighter green, turn the heat down to low. when it starts to turn a darker color, remove it. continue cooking the butter and peppercorns until the the butter underneath the foam on top is brown, approximately 8-10 minutes. strain the peppercorns out and allow the butter to slightly cool. no need to clean the pan, you can use it in a later step.
3. add 2 tablespoons of the brown butter to a large cast iron pan and turn it on to medium heat. add the bison and use a spatula to break it up into smaller pieces. cook until mostly brown. it is okay for it to not be fully cooked because it will go into the oven, but you want it most of the way there. remove the bison to a paper towel-lined plate. 
4. use a paper towel to remove most of the liquid in the pan. 
5. add 3 tablespoons of the brown butter to the pan. turn it on high heat and add the mushrooms. cook for five minutes, stirring occasionally, then remove them and any liquid in the cast iron to the empty pot that you cooked the brown butter in. 
6. with the stove now on low heat, add a tablespoon of the butter, 2/3 cup of beef broth, 1 tablespoon of the soy sauce, 2 tablespoons of Worcestershire sauce, and a half teaspoon of salt to the pan. add the onions, and cook for 7 minutes. after 10 more minutes, turn the heat up to medium high heat until the onions get darker in color. remove to the same pan as the mushrooms. 
7. add a teaspoon of the brown butter to the pan and turn it up to medium-high. let it be until the pan starts smoking. add the carrots, and spread them out in an even layer. don't disturb them too much in order to slightly char the carrots. remove them from the pan to the pot with the mushrooms and onions. 
8. using the residual heat in the cast iron, add one more tablespoon of the butter and the bell peppers. cook until the peppers are softened, approximately 3 minutes. crank the heat up to high and sauté in order to evaporate some of the liquid in the pan. this will also take approximately 3 minutes. 
9. in the bowl of the stand mixer, add the potatoes, the rest of the brown butter (excluding any brown bits of milk solids on the bottom of the bowl), the sour cream, 3/4 teaspoon salt, 2 tablespoons of beef broth, dried chives, one tablespoon garlic powder, onion powder, and onion salt. mix until completely combined.
10. put the rest of the beef broth, soy sauce, and Worcestershire sauce in the cast iron and warm on medium-low until it starts to release steam. remove 1/2 cup of it to a separate bowl and whisk the cornstarch into the 1/2 cup. add that back to the pan once the cornstarch has completely dissolved. cook until slightly thickened, then add the bison, vegetables, remaining garlic powder, pepper and a teaspoon of salt to it.
11. allow everything to slightly cool. if making ahead, you could store the potatoes and the meat filling mixture separately for up to 3 days. 
12. preheat the oven to 400 degrees (F). 
13. once the potatoes have cooled, add the egg yolks. 
14. either pipe or smooth the potatoes on top of the meat mixture. you don't want it completely smooth in order for there to be some browned parts and some parts still fluffy and creamy at the end. 
15. bake for 40 minutes-1 hour. if you notice the tops getting too brown, turn the heat down to 350.