Preheat your oven to 375°F (190°C). In a mixing bowl, whisk together the eggs, milk, dried thyme, salt, and black pepper until smooth and slightly frothy.
Heat olive oil in an oven-safe skillet over medium heat until shimmering. Add the diced pumpkin and chopped bell pepper, sautéing until they begin to soften and become fragrant, about 5 minutes.
Add the chopped spinach to the skillet and cook until just wilted, about 2 minutes. Stir everything to evenly distribute the vegetables.
Pour the beaten egg mixture evenly over the sautéed vegetables in the skillet. Sprinkle grated cheese on top if using.
Transfer the skillet to the preheated oven and bake until the frittata is puffed, set in the center, and lightly golden on top, about 15–20 minutes.
Remove the skillet from the oven and let the frittata cool slightly before slicing. The final result should be fluffy, tender, and colorful with a slightly crispy top.