Step 1

Preheat the oven to 400°F.

Step 2

Heat the oil in a large nonstick skillet over medium heat. Add the onion and sauté until soft and translucent, about 4 to 5 minutes. Add the diced eggplant and sliced mushrooms and sauté, stirring occasionally, until they have released all of their moisture and have cooked down. Continue to cook until the liquid in the pan has evaporated and the vegetables are starting to brown on the edges, about 10 to 15 minutes. Add the oregano, crushed red pepper flakes, if using, and salt and pepper. Sauté 1 to 2 minutes, until fragrant.

Step 3

While the vegetables are sautéing, combine the ricotta and 1/3 cup milk in a mixing bowl. Stir well to combine. The mixture should be really creamy and easy to stir. If it seems a little dry, add a few tablespoons of extra milk until creamy. Stir in the chopped fresh spinach. Set aside.

Step 4

Pour 1 cup of sauce into the bottom of the baking dish and spread out evenly.

Step 5

Place 4 lasagna noodles in one even layer over the sauce. It's ok if they overlap slightly. If you have gaps on the sides or ends, you can break a noodle and fill in. However, make sure you have at least 8 full noodles left for the remaining two layers.

Step 6

Spread half of the spinach ricotta mixture over the noodles as evenly as possible. Spoon half of the eggplant mushroom mixture over the ricotta and spread out as best as you can.Repeat the layers again. About 1 1/2 cups of sauce, 4 noodles (or more to fill in gaps), the remaining spinach ricotta mixture and then the remaining eggplant mushroom mixture.

Step 7

Place the final 4 noodles (again filling in gaps if necessary if you have noodles left). Cover with 1 1/2 cups of sauce. Sprinkle the shredded mozzarella evenly over the top. Cover with aluminum foil and bake for 20 minutes. Uncover and bake an additional 10 minutes until the noodles are tender and easily pierced with a knife and the cheese is melty.

Step 8

Place the lasagna under the broiler for a minute or two if you want to brown the cheese. This is optional.

Step 9

Let the lasagna rest for at least 20 minutes before slicing and serving. Garnish with fresh basil leaves, if desired.