Preheat oven to 375 degrees. In a large pot of boiling salted water, cook pasta shells according to package instructions. Drain well and arrange in a single layer on an oiled rimmed baking sheet. Meanwhile, in a medium bowl, combine ricotta, 1/2 cup Parmesan, and lemon zest. Stir in spinach and season filling to taste with salt and pepper.
Transfer filling to a large zip-top plastic bag. Twist and squeeze bag so filling is in one corner. With scissors, snip a 3/4-inch opening in corner. Pipe filling into both ends of each pasta shell. Arrange stuffed shells in an 8-inch square glass baking dish.
In a medium bowl, whisk together marinara sauce, oregano, and cream. Pour sauce evenly over stuffed shells and top with 1/2 cup Parmesan. Bake until sauce in center of dish is bubbling and Parmesan is browned in spots, about 30 minutes. Let sit 5 minutes before serving.
Swap it: Use 1/2 pound peeled, deveined shrimp, finely chopped, instead of the spinach.