Vegetarian Lasagna With Easy Roasted Tomato Sauce

Preheat oven to 425°. Mix together 1 small onion, thinly sliced, 3 garlic cloves, thinly sliced, two 28-oz. cans whole peeled tomatoes, ½ cup extra-virgin olive oil, 1 Tbsp. plus 1½ tsp. sugar, 1 Tbsp. Diamond Crystal or 1¾ tsp. Morton kosher salt, and ½ tsp. crushed red pepper flakes in a 13x9" or 3-qt. baking dish. Bake, stirring halfway through, until sauce is bubbling, tomatoes break easily when pressed, and onion is translucent, 40–50 minutes.

Let cool slightly, then break up tomatoes with a fork to form a chunky sauce. (If you prefer a smoother sauce, blend with an immersion blender or in a regular blender or food processor.) Taste and season with more salt if needed. Do ahead: Sauce can be made 5 days ahead. Let cool; cover and chill, or freeze up to 3 months in an airtight container.

Preheat oven to 350°. If using 1 lb. dried lasagna noodles, cook in a large pot of boiling salted water, stirring occasionally and separating to keep from sticking together, until al dente; drain.

Meanwhile, mix two 10-oz. packages frozen spinach, thawed, squeezed dry, or 1½ cups chopped cooked greens, 2 Tbsp. extra-virgin olive oil, and 1 tsp. Diamond Crystal or ½ tsp. Morton kosher salt in a medium bowl to combine.

Stir together ½ cup tomato sauce, remaining 2 Tbsp. extra-virgin olive oil, and ¼ cup water in same (no need to clean) or another 13x9" or 3-qt. baking dish. Arrange a single layer of noodles from cooked noodles or 1 lb. fresh pasta sheets (8–10 9x4" sheets) over sauce in dish. Ladle 1¼ cups tomato sauce over noodles and spread into an even layer. (Some water may rise up and mix with the sauce; this is fine.) Dollop tablespoonfuls of ⅔ cup whole-milk ricotta evenly over sauce. (You don’t need to spread them out; this will keep each bite interesting.) Top with one third of spinach mixture. Sprinkle one quarter of 2 oz. Pecorino Romano or Parmesan, finely grated, evenly over. Layer one quarter of 1 lb. sliced provolone cheese on top. Starting with noodles repeat layers 2 more times. Top with another layer of noodles, then evenly spread 1½ cups tomato sauce over and sprinkle remaining Pecorino Romano and remaining provolone cheese on top. Press a sheet of parchment paper directly onto surface of lasagna, then cover baking dish with foil and crimp tightly to seal.

Place lasagna on a baking sheet to catch any drips and bake until center is warmed through and cheese is beginning to melt, about 1 hour and 20 minutes. Remove foil and parchment paper and continue to bake lasagna until cheese is starting to brown and sauce is bubbling around the edges, 25–35 minutes longer. Let cool at least 15 minutes before serving. Do ahead: Lasagna can be assembled 2 days ahead. Cover and chill.