Melt the butter in a pan over high heat until it bubbles actively. The pan should sound lively and hot enough to brown the beef quickly.

Sear the beef for 2-3 minutes, turning it until every side has developed a deep brown crust. Keep the center uncooked at this stage; the color should stay on the surface.

Brush the browned beef all over with Dijon mustard, then set it aside to cool while leaving the butter in the pan. The mustard should form a thin, even coating over the warm beef.

Reduce the heat to medium-high and add the garlic, onion, mushrooms, and thyme to the pan. Stir as the mushrooms release moisture and the thyme becomes fragrant.

Cook the mushroom mixture for about 10 minutes until the pan is dry and no liquid remains. The finished duxelles should look like a dense, crumbly paste rather than a wet sauté.

Lay plastic wrap on the counter and overlap the 10 prosciutto slices into a square. Keep the slices slightly overlapped so they form one continuous layer.

Spread the cooled mushroom mixture across the prosciutto, then place the mustard-coated beef in the center. Leave the edges clear enough for the prosciutto to fold over the roast.

Use the plastic wrap to roll the prosciutto tightly around the beef until it is fully enclosed. The wrapped roast should hold a compact log shape with no exposed beef.

Melt the mozzarella in the microwave or on the stove. The cheese should be fully melted and stretchy before the dry ingredients are added.

Mix in 3/4 cup (75g) almond flour and the baking powder, then knead until a dough starts to form. Work while the mozzarella is warm so the mixture becomes cohesive instead of crumbly.

Knead in the egg until it is fully incorporated. The dough will become softer and more uniform as the egg disappears into the cheese mixture.

Add the remaining almond flour and knead until the dough is smooth. Stop once it holds together and feels flexible enough to roll.

Roll the dough flat between two sheets of parchment paper. A uniform sheet helps the crust bake evenly around the beef.

Place the prosciutto-wrapped beef in the center of the dough and bring the pastry around it, sealing the edges. Pinch the seam closed so the filling is enclosed without large gaps.

Turn the Wellington seam-side down and brush the outside with beaten egg. The smooth side should face up for the most even browning.

Bake at 400°F / 200°C for 45 minutes, covering loosely with foil after 20 minutes. The foil should protect the crust if it is browning quickly while the beef continues cooking.

Rest the baked Wellington for about 10 minutes before slicing. The crust will settle and the meat juices will stay contained more easily during carving.