Make the sauce: Pulse tomatoes with
juice in a food processor until finely
chopped. Heat oil in a large straight-sided
skillet over medium heat. Cook
onion and 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 oregano and salt. Let cool.
Make the lasagna: 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 oil.
Dot with remaining 1/4 cup ricotta. Season
with pepper. Bake, uncovered, until
lasagna bubbles and top browns,
50 to 60 minutes. Let stand for 10 minutes.
Garnish with oregano.