Heat oven to 375°F and lightly oil a 9x13-inch baking dish.
Bring a large pot of salted water to a boil.
Cook shells until just shy of al dente, 8–9 minutes. Drain and rinse cool.
Toss shells with olive oil to prevent sticking; set aside.
Mix pumpkin, ricotta, spinach, mozzarella, Parmesan, garlic, milk, sage, nutmeg, salt, and pepper until creamy.
Spread 1 cup marinara in the bottom of the prepared dish.
Fill each shell with a heaping spoonful of filling. Nestle seam up in the sauce.
Spoon remaining marinara over shells. Sprinkle remaining mozzarella and Parmesan on top.
Cover with foil and bake 20 minutes. Uncover and bake 10–15 minutes more until bubbly.
Rest 5 minutes before serving.
Notes
Variation: Swap marinara for a light cream sauce and add a pinch of smoked paprika. For a heartier bake, layer sautéed mushrooms under the shells. Make-ahead: Assemble up to 24 hours in advance and refrigerate; add 5–10 minutes to bake time. Freezer tip: Freeze the assembled, unbaked dish for up to 2 months; thaw overnight and bake as directed.This recipe is an original creation inspired by classic Easy Pumpkin Stuffed Shells flavors. All ingredient ratios and instructions are independently developed.