Step 1: Preheat your oven to 400°F (200°C). Toss the pumpkin cubes with a tablespoon of olive oil and a pinch of salt. Roast on a baking sheet for 15-20 minutes until tender.
Step 2: While pumpkin is roasting, heat the remaining olive oil in a 10-inch oven-safe skillet over medium heat. Add the onion and sauté for 3-4 minutes.
Step 3: Add the sliced mushrooms to the skillet. Cook for 6-8 minutes until the moisture has evaporated and the mushrooms are browned.
Step 4: Stir in the garlic and dried thyme, cooking for another minute until fragrant. Add the spinach and cook just until wilted.
Step 5: In a large bowl, whisk together the eggs, milk, salt, and pepper until well combined.
Step 6: Reduce the oven heat to 375°F (190°C). Evenly distribute the roasted pumpkin in the skillet with the other vegetables.
Step 7: Pour the egg mixture over the vegetables in the skillet. Sprinkle the cheese evenly over the top.
Step 8: Bake the pumpkin mushroom and spinach frittata for 15-20 minutes, or until the eggs are set and the top is lightly golden. Let rest for 5 minutes before slicing.