Boil the shells: Bring a large pot of salted water to a boil. Cook the jumbo shells until just shy of al dente (about 1 minute less than package directions). Drain and spread on a sheet pan to cool so they don’t stick.

Sauté aromatics: Heat olive oil in a large skillet over medium. Add onion and a pinch of salt. Cook 3–4 minutes until softened, then stir in garlic for 30 seconds.

Brown the chicken: Add ground chicken, breaking it up with a spoon. Season with salt, pepper, Italian seasoning, and red pepper flakes if using. Cook until no longer pink and lightly browned, 6–8 minutes.

Add greens: Stir in chopped spinach and cook until wilted, 1–2 minutes. Take the skillet off the heat and let the mixture cool slightly.

Make the filling: In a large bowl, combine ricotta, 1 cup mozzarella, Parmesan, egg, chopped basil or parsley, and the cooled chicken mixture. Taste and adjust salt and pepper. The filling should be flavorful and slightly thick.

Prep the baking dish: Spread about 1 to 1 1/2 cups marinara on the bottom of a 9x13-inch baking dish to prevent sticking and keep shells moist.

Stuff the shells: Spoon a generous heaping of filling into each shell (about 2 tablespoons). Arrange stuffed shells snugly in the dish.

Sauce and cheese: Pour remaining marinara evenly over the shells. Sprinkle the remaining 1/2 cup mozzarella on top, plus a little extra Parmesan if you like.

Bake: Cover the dish with foil and bake at 375°F (190°C) for 20 minutes. Uncover and bake 10–15 minutes more, until the sauce is bubbling and the cheese is melted and lightly golden.

Finish and serve: Let rest 5–10 minutes to set. Garnish with more basil or parsley. Serve warm with a simple salad or garlicky bread.