Heat the oven to 375° F. Set a rack in the center so the pan bakes evenly from edge to edge.

Warm the olive oil in a large skillet or Dutch oven over medium-high heat, then cook the onion, bell pepper, mushrooms, and zucchini for 7 to 8 minutes, stirring now and then; add the garlic and cook for 30 seconds. The vegetables should be softened with light browning, and the lively sizzle should quiet as excess moisture cooks away.

Stir the marinara, Italian seasoning, salt, pepper, crushed red pepper flakes, and parsley into the vegetables; bring the mixture to a simmer, reduce to low, and cook for 10 to 15 minutes before folding in the spinach. The sauce should look thicker around the spoon, smell fragrant with herbs and garlic, and wilt the spinach without turning it dull.

Beat the egg lightly in a medium bowl, stir in the ricotta, and shred the mozzarella while the sauce simmers. The ricotta mixture should be evenly blended and loose enough to drop from a spoon in small dollops.

Spread about 1 cup of sauce across the bottom of a 9×13-inch baking dish, arrange 3 noodles over it, then add ⅓ of the remaining sauce, ½ of the ricotta mixture, ⅓ of the mozzarella, and ⅓ of the Parmesan. The sauce underneath cushions the first noodles and prevents dry corners from forming.

Build the second layer with 3 noodles, ½ of the remaining sauce, the remaining ricotta, ½ of the remaining mozzarella, and ½ of the remaining Parmesan; finish with the last 3 noodles, remaining sauce, mozzarella, and Parmesan. Scatter the ricotta in small, even dollops so every serving gets creamy filling rather than one large pocket.

Cover the dish with foil sprayed with cooking spray on the underside. The oiled surface keeps the melted cheese from clinging to the foil when it is removed.

Bake covered for 35 minutes, with a rimmed baking sheet on the rack below to catch drips; uncover and bake for 10 to 15 minutes more, then rest the lasagna for 15 minutes before serving. The edges should bubble, the cheese should be lightly browned, and the bubbling sauce should settle along the pan edges during the rest.