Preheat the oven to 200°C.
Squeeze the liquid from the grated potatoes using a clean tea towel.
Combine the potatoes, crème fraîche, and egg yolk in a bowl and season.
Fry the mixture in 25g butter using 7.5cm rings for 4 minutes until brown.
Flip the rosti and cook for another 4 minutes until cooked through.
Roast halved shallots on 25g butter and thyme for 10 minutes until soft.
Sauté chopped shallot and garlic in 55g butter, then reduce with Madeira and stock.
Stir in tarragon and the remaining 30g butter for a glossy finish.
Sear beef fillets in rapeseed oil for 4 minutes per side.
Add mushrooms to the beef pan, basting the meat as it finishes.
Stir parsley into the mushrooms and cook for 2 more minutes.
Wilt the spinach in a separate pan with two tablespoons of water.
Layer the rosti with mushrooms, spinach, and beef, then drizzle with sauce.