Preheat the oven to 375F.

Trim and roughly chop the mushrooms. Working in batches, use a food processor to mince the mushrooms to small, uniform pieces, being careful not to overprocess. Alternatively, mince the mushrooms using a Chef Knife.

Heat a large Stainless Clad Saute Pan over high heat. Add 2 Tbsp. olive oil, then the mushrooms. The pan will be very full but as the mushrooms cook down, they will become easier to maneuver. Cook, stirring often with a spatula, until mushrooms are dark brown and caramelized, 10–15 minutes.

Create a well in the center of the pan and add the remaining 1 Tbsp. oil along with onion and garlic. Stir to coat with oil, then incorporate into the mushroom mixture. Continue to cook, stirring, until the onions are just translucent, 2–3 minutes. Add thyme and season with salt and pepper. Continue to cook, stirring occasionally, until almost all the moisture has evaporated, 4–5 minutes.

Add tomato paste and cook, stirring and pressing the mixture with spatula, until it turns a deep crimson color, about 2 minutes. Add the stock to the mushroom mixture and allow to reduce until thickened and almost dry (but not sticking to pan), 20–25 minutes.

Transfer mixture from the saute pan to a medium bowl set over another bowl of ice water. Stir the mixture to release steam and cool to room temperature. Transfer bowl with mushroom mixture to refrigerator to chill until ready to use.

Bring a Stock Pot full of water to a boil over high heat. Once boiling, add a generous amount of salt and the lasagna noodles and cook according to package directions until al dente.

Add ricotta cheese to the cooled mushroom mixture. Taste and season with salt. Add eggs and stir together until combined.

Add a thin layer of Parmesan to a 10x6.6” Oval Gratin Baking Dish. Layer the lasagna starting with a third of the pasta, followed by a third of the mushroom filling. Start another layer with a third of the mozzarella and Parmesan, a third of the pasta, and another third of the filling.

Repeat with cheeses, remaining pasta, and remaining mushroom filling. Press any pasta edges down into the baking dish (it will be very full). Top the lasagna with remaining mozzarella and Parmesan.

Place the baking dish on a Sheet Pan to catch any drips. Bake, uncovered, until the filling is bubbling and the cheese has started to brown, about 40–45 minutes. Let the lasagna sit for at least 10 minutes before cutting and serving.