Position rack in middle of oven; preheat to 375°F. Butter a lasagna pan.

In a large saute pan over medium heat, warm 1 Tbs. olive oil. Working in batches, add zucchini, season with salt and pepper and cook until golden brown, 2-3 min. per side. Transfer to a platter and repeat with remaining zucchini; set aside.

In a bowl, combine ricotta, Parmigiano-Reggiano, tarragon, salt and pepper and stir to combine; set aside.

In a saucepan over medium heat, melt butter. Add flour and whisk until smooth, then cook until fragrant, 2-3 min. Slowly whisk in milk until smooth. Cook, whisking occasionally, until thickened, 3-5 min. Season with salt, pepper and nutmeg; set aside.

Cover the bottom of the lasagna pan with a layer of lasagna noodles. Spread one-third of the ricotta mixture over the noodles. Spread one-third of the tomatoes over the ricotta mixture, then top with one-third of the zucchini. Pour one-third of the sauce over the zucchini. Repeat the layering twice, finishing with a layer of noodles. Sprinkle with mozzarella cheese.

Bake until golden and bubbling, about 50 min. Let stand for 10 min. before serving.