Step 1: Preheat your oven to 400°F (200°C). Toss the cubed pumpkin with 1 tablespoon of olive oil, salt, and pepper. Spread on a baking sheet and roast for 15-20 minutes until tender. Lower oven temperature to 375°F (190°C).
Step 2: In a 10-inch oven-safe skillet, heat the remaining tablespoon of olive oil over medium heat. Add the mushrooms and cook until browned, approximately 5-7 minutes.
Step 3: Add the red onion to the skillet and sauté for 3 minutes until translucent. Stir in the garlic and cook for another minute until fragrant.
Step 4: Add the roasted pumpkin and fresh spinach to the skillet. Stir gently until the spinach just begins to wilt.
Step 5: In a medium bowl, whisk together the eggs, heavy cream, salt, and pepper. Pour the mixture into the skillet over the vegetables.
Step 6: Sprinkle the feta cheese (if using) evenly over the top. Cook on the stovetop for 2-3 minutes until the edges are beginning to set.
Step 7: Transfer the skillet to the preheated oven and bake for 12-15 minutes, or until the center is set and the top is lightly golden.
Step 8: Let the pumpkin mushroom and spinach frittata rest for 5 minutes before slicing and garnishing with fresh parsley.