Preheat oven to 375°F.

Bring a large pot of well-salted water to a boil. Cook the lasagna noodles until just before al dente, according to package directions, stirring often. Drain.

Drizzle a baking sheet with olive oil. Lay the drained noodles flat and turn them so both sides are lightly coated with oil to prevent sticking.

Mince the garlic. In a medium saucepan, melt butter over medium heat. Add garlic and sauté until fragrant, about 1–2 minutes.

Turn the heat down and carefully add crushed tomatoes, tomato sauce, dried basil, dried tarragon or oregano, 3/4 teaspoon kosher salt, and a few grinds of black pepper. Be cautious — hot tomato sauce can spatter briefly.

Remove 1/4 cup of the sauce into a small bowl. Stir the cornstarch into that portion until smooth, then whisk it back into the saucepan to thicken the sauce. Simmer gently on low for at least 15 minutes, then remove from heat when ready to assemble.

In a large skillet, place the baby spinach and 1/4 cup water. Cook, stirring often, until fully wilted and reduced (about 3 minutes). If needed, cook in batches. Sprinkle with two pinches of kosher salt. Remove from heat and let cool slightly.

Once cool enough to handle, squeeze out all excess liquid from the cooked spinach with your hands or a clean towel. Roughly chop and set aside.

Destem and chop the thyme. Put 1½ tablespoons chopped thyme into a medium bowl, and save about 1/2 tablespoon for the top of the lasagna.

Add lemon zest, nutmeg, ricotta, 3/4 cup Parmesan, 2 cups shredded mozzarella, 1 teaspoon kosher salt, and several grinds of black pepper to the thyme. Stir until evenly combined.

In a 9" x 13" baking dish, spread 1/2 cup of the tomato sauce on the bottom.

Layer: noodles (single layer), half the ricotta mixture (drop in dollops and spread), half the chopped spinach, and about 1 cup tomato sauce.

Repeat: another layer of noodles, remaining ricotta mixture, remaining spinach, and 1 cup sauce.

Top with a final layer of noodles. Pour the remaining 1½ cups sauce over the top. Sprinkle with the remaining 1 cup mozzarella, 1/4 cup Parmesan, and the reserved 1/2 tablespoon thyme.

Cover tightly with aluminum foil and bake for 40 minutes. Remove the foil carefully and bake another 10 minutes until bubbling and lightly browned.

Let the lasagna rest for 15 minutes before slicing so it sets. Serve warm.