Preheat your oven to 375°F (190°C) and lightly grease your oven-safe skillet or baking dish.
Dice the seasonal squash into small, even pieces and chop the fresh spinach and herbs.
Heat olive oil in the skillet over medium heat, then add the diced squash and cook for about 5 minutes until they start to soften and turn golden.
Add the diced bell pepper to the skillet and cook for another 2-3 minutes until fragrant and slightly tender.
Stir in the chopped spinach and herbs; cook just until the spinach wilts down, about 1 minute, then remove from heat.
In a mixing bowl, whisk the eggs thoroughly and season with salt and pepper.
Pour the beaten eggs over the cooked vegetables in the skillet, spreading evenly with a spatula.
Sprinkle the shredded cheddar cheese on top of the egg mixture.
Transfer the skillet to the preheated oven and bake for 15-20 minutes until the eggs are set and the top is lightly golden.
Remove the skillet from the oven and let the frittata rest for a few minutes to settle before slicing.
Slice the frittata into wedges and serve warm, garnished with extra herbs if desired.
