- Bring a large pot of salted water to a boil and cook the pasta shells until just al dente, about 8 minutes. Drain and set aside. 
- Heat olive oil in a skillet over medium heat. Add chopped garlic and onions, sautéing until fragrant and translucent, about 3 minutes. The kitchen will fill with a warm, savory aroma. 
- In a mixing bowl, combine ricotta, shredded mozzarella, chopped spinach, and a pinch of nutmeg. Season with salt and pepper, then mix until smooth and creamy. 
- Using a spoon or piping bag, carefully stuff each cooked shell with the cheese mixture, filling them generously but avoiding overstuffing to prevent spillage. 
- Arrange the stuffed shells in a baking dish, placing them seam side down for stability. Pour marinara sauce evenly over the shells, covering them completely. 
- Sprinkle extra mozzarella on top and add a dash of red pepper flakes if you like a spicy kick. This topping will melt and turn golden as it bakes. 
- Bake uncovered in a preheated oven at 180°C (350°F) for 25-30 minutes, until bubbling and golden brown on top. The sauce should be thick around the edges, and the cheese melted and slightly crispy. 
- Remove from the oven and let rest for a few minutes. The shells will be tender yet firm, with gooey cheese oozing from each bite, ready to serve.