1Preheat 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.
2Let 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.
3Preheat 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.
