Preheat an oven to 375ºF. Butter a lasagna pan. In a large sauté pan over medium heat, warm 1 Tbs. of the olive oil. Working in batches, add the zucchini, season with salt and pepper and cook until golden brown, 2 to 3 minutes per side. Transfer to a platter and repeat with the remaining zucchini; set aside.
In a bowl, combine the ricotta, Parmigiano-Reggiano, tarragon, salt and pepper and stir to combine; set aside.
In a saucepan over medium heat, melt the butter. Add the flour and whisk until smooth, then cook until fragrant, 2 to 3 minutes. Slowly whisk in the milk until smooth. Cook, whisking occasionally, until thickened, 3 to 5 minutes. Season with salt, pepper and nutmeg; set aside.
Cover the bottom of the prepared 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 the mozzarella cheese. Bake until the lasagna is golden and bubbling, about 50 minutes. Let stand for 10 minutes before serving. Serves 10 to 12.