Preheat the oven to 350°F. Start boiling the potatoes for Step #4. Slice the mushrooms and briefly fry them in vegetable oil before removing them from the pan and setting aside.
Cut the beef into small pieces and fry them in the mushroom pan. Season with salt and pepper and add the sliced onions and mustard. Let everything cook for about 10 minutes and then add the beef broth. Let everything simmer until the meat has softened.
Add the mushrooms back to the pan, along with the crème fraîche. Stir everything together and transfer to a casserole dish.
Add the shredded cheese and egg yolk to your cooked potatoes and mash everything together until smooth. Spread out the mashed potatoes on a piece of parchment paper and mold it into the shape of the top of your casserole dish. Use the parchment paper to flip the mashed potato lid on top of the casserole.
Slice a red, yellow, and green bell pepper and arrange the slices on the mashed potato lid. Cook for 30 minutes.
