Step 1

In a large pan, over medium heat, heat 2 tablespoons of olive oil. Add the mushrooms and cook, stirring, until softened and all the moisture has disappeared (about 6-8 minutes). Transfer the mushrooms to the plate and set aside.

Step 2

In the same pan, heat the remaining tablespoon of olive oil. Add the onions, celery, and carrots. Cook until the vegetables begin to soften, about 10 minutes. Add the garlic and cook for 1 minute more.

Step 3

Return the mushrooms to the pan. Stir in the canned tomatoes, Italian seasoning, salt, and black pepper. Bring to a boil, cover, and simmer over low heat for about 10 minutes.

Step 4

Melt the butter in a small saucepan. Stir in the flour and cook, stirring constantly, for 2-3 minutes until smooth (but don’t let it brown).

Step 5

Gradually stir in the milk. Add the nutmeg and salt, then stir over low heat for about 3-4 minutes, until thickened.

Step 6

Preheat the oven to 350°F (175°C).

Step 7

Use a mandolin to slice zucchini lengthwise into thin sheets, about 1/8 inch (0.3cm) thick.

Step 8

Spread half of the bechamel sauce on the bottom of a 9x13 inch baking dish. Arrange a layer of zucchini slices. Spread half of the mushroom Bolognese sauce. Top with another layer of zucchini slices.

Step 9

Top with half of the shredded mozzarella followed by the rest of the mushroom Bolognese. Arrange a last layer of zucchini slices topped with the rest of the bechamel sauce. Finally, sprinkle with the remaining mozzarella.

Step 10

Bake 40 minutes. Garnish with parsley, if desired.