Place dried porcini mushrooms in a heatproof bowl and cover with hot or boiling water; soak for 20–30 minutes, then reserve the soaking liquid and drain the mushrooms.
Make the Easy 20 Minute Marinara or measure 3 cups of your favorite marinara and set aside.
Preheat the oven to 350°F (175°C).
Using a mandolin or sharp knife, slice zucchinis lengthwise into about 1/8-inch thick slices; sprinkle slices with kosher salt and let sit for 10 minutes to draw out moisture.
Blot zucchini slices with paper towels to remove excess moisture and salt. Arrange in a single layer on a lined baking sheet and bake for 10 minutes. Remove and set aside.
Heat olive oil in a sauté pan over medium heat with a pinch of red pepper flakes and the garlic until fragrant, about 30–60 seconds.
Add the diced onion (and any reserved diced zucchini pieces) and season with salt; sauté about 5 minutes until starting to soften.
Add the crimini and shiitake mushrooms, dried oregano, dried thyme, and salt and pepper; sauté until the mushrooms release their liquid and begin to brown, about 10–15 minutes.
Stir in the rehydrated porcini mushrooms and about 1/4 cup of the reserved porcini soaking liquid; continue to cook until the liquid evaporates and the mushroom mixture is nicely browned.
To assemble the lasagna in a 9x13 baking dish: spread about 1/4 cup marinara on the bottom. Layer half the zucchini slices over the sauce, spread half the mushroom mixture over the zucchini, spoon half the remaining marinara over the mushrooms, then sprinkle half the Parmesan and half the shredded mozzarella.
Repeat the layers once more (zucchini, mushroom mixture, marinara, Parmesan, mozzarella) to finish the lasagna.
Bake uncovered until the cheese is browned and bubbly, about 40 minutes; if the top browns too quickly, tent with foil, or broil 1–2 minutes at the end to achieve more browning if needed.
Remove from oven and let rest for a few minutes before cutting. Garnish with chopped basil and extra Parmesan if desired.
