Pulse tomatoes with juice in a food processor until finely chopped. Heat oil in a large straight-sided skillet over medium heat.
Cook 1 cup diced onion and 1/4 teaspoon red-pepper flakes, stirring occasionally, until onion is tender, about 8 minutes.
Add turkey; cook,breaking up any large pieces, until browned, 3 to 4 minutes.
Add tomatoes; bring to a boil. Reduce heat; simmer until thick, about 20 minutes.
Stir in 2 teaspoons dried oregano or 2 tablespoons fresh oregano and salt. Let cool.
Preheat oven to 375 degrees.
Slice zucchini lengthwise into thin strips (about 1/8 inch thick) using a mandoline or a sharp knife.
Place 5 or 6 zucchini slices, overlapping slightly, in the bottom of an 8-inch square baking dish. Top with 1 cup sauce. Dot with 1/4 cup ricotta.
Repeat twice with zucchini, remaining sauce, and 1/2 cup ricotta, alternating direction of zucchini.
Top with remaining zucchini, alternating direction; brush with very small amount of olive oil. Dot with remaining 1/4 cup ricotta.
Season with fresh ground pepper.
Bake, uncovered, until lasagna bubbles and top browns, 50 to 60 minutes. Let stand for 10 minutes.
Garnish with fresh oregano.