In a large pot of boiling, salted water, cook the shells according to package directions. Drain and rinse the shells under cold water; set aside.
In large skillet, cook the onions in olive oil until they have softened (3 to 5 minutes). Set aside.
In a food processor, pureé the cottage cheese until smooth. Scrape the cottage cheese into a large bowl. Add the breadcrumbs, nutmeg and ½ cup of the Parmesan cheese. Mix thoroughly. Stir in the onions and spinach, and season with salt and pepper. Stir in the egg white.
Stuff each shell with about 2 tablespoons of the cheese/spinach mixture.
Spread 1 cup of the marinara sauce in the bottom of a 9x13-inch baking dish. Arrange the stuffed shells in a single layer on top of the sauce and spoon the remaining marinara sauce over the top. Sprinkle with the remaining Parmesan cheese.
Bake for 30 minutes, until the cheese is golden and the shells are heated through. If the top is browning too quickly, tent loosely with foil. Cool 10 minutes before serving.