Cook the shells: Bring a large pot of salted water to a rolling boil. Cook the jumbo shells until just al dente, 1–2 minutes less than the package suggests. Drain, rinse with cool water, and spread on a baking sheet so they don’t stick.

Wilt the spinach: Heat olive oil in a large skillet over medium. Add spinach with a pinch of salt and cook until just wilted, 2–3 minutes. Transfer to a cutting board, cool slightly, then chop and squeeze out excess liquid. If using frozen, thaw completely and wring dry in a clean towel.

Make the Alfredo base: In a medium saucepan, melt butter over medium-low heat. Add garlic and cook 30–60 seconds until fragrant but not browned. Pour in heavy cream, bring to a gentle simmer, and cook 3–4 minutes, stirring.

Add Parmesan: Lower the heat and whisk in 1 cup Parmesan until smooth. Season with salt, pepper, and a tiny pinch of nutmeg. The sauce should lightly coat the back of a spoon.

Mix the filling: In a large bowl, combine ricotta, 1 cup mozzarella, chopped spinach, and 1/2 cup of the warm Alfredo sauce. Stir until creamy. Taste and season with salt and pepper.

Prep the baking dish: Spoon about 1 cup of the Alfredo sauce into the bottom of a 9x13-inch baking dish and spread it evenly.

Fill the shells: Spoon 1–2 tablespoons of the spinach-ricotta mixture into each shell and arrange them snugly in the dish, open side up.

Top with sauce and cheese: Pour the remaining Alfredo over the shells, aiming for even coverage. Sprinkle the remaining mozzarella and 1/4 cup Parmesan on top.

Bake: Cover loosely with foil and bake at 375°F (190°C) for 15 minutes. Remove the foil and bake another 10–12 minutes, until the cheese is melted and lightly golden at the edges.

Rest and garnish: Let the shells rest for 5–10 minutes so the sauce settles. Finish with chopped parsley or basil and a crack of black pepper.