Step 1: Preheat your oven to 375°F (190°C). Toss the pumpkin cubes with olive oil, salt, and a pinch of nutmeg on a baking sheet. Roast for 20 minutes or until tender. Remove and set aside.
Step 2: While the pumpkin roasts, place the pie crust in a 9-inch pie plate. Line with parchment and pie weights, and blind-bake for 12 minutes. Remove weights and bake for another 3 minutes. Set aside.
Step 3: In a large skillet over medium heat, sauté the diced onion until translucent. Add the garlic and spinach, cooking until the spinach is fully wilted. Squeeze out any excess liquid and set aside.
Step 4: In a medium bowl, whisk together the eggs, heavy cream, milk, remaining nutmeg, salt, and pepper until well combined.
Step 5: Layer the roasted pumpkin, wilted spinach, and shredded Gruyère cheese into the bottom of the pre-baked pie crust.
Step 6: Carefully pour the egg custard over the fillings. Place the quiche in the oven and bake for 35-40 minutes, or until the custard is set but slightly jiggly in the center.
Step 7: Let the pumpkin and spinach quiche cool for at least 15 minutes before slicing and serving.