For the Boeuf Bourguignon with Parsley Mousseline, heat the butter and oil in a pan and sear the beef flaps on both sides. Peel the onions, cut them into half rings and fry them for about 2 minutes.
Deglaze with some of the red wine and add the tomato paste. Peel the carrots, cut them into rough slices and add them together with the bay leaves, chopped parsley, chopped garlic and bacon cubes.
Clean the mushrooms, slice them and add them. Cover the whole with the remaining red wine and bring to a boil briefly. Then lower the temperature, put the lid on and let it simmer for about 3.5 hours.
Stir from time to time and season with salt and pepper if necessary. For the parsley mousseline, peel and wash potatoes, cook until done and let steam. Peel and finely chop onions and garlic, heat oil in a pot and sauté onion and garlic.
Deglaze with white wine and bring to a boil. Then reduce the temperature, add whipping cream and butter and puree the whole thing with a hand blender. Finely mash the potatoes, add the cream mixture and mash again.
Add the egg yolk and the chopped parsley and stir everything into a creamy mixture. Season the Boeuf Bourguignon with Parsley Mousseline with salt and pepper!