Melt 1 tbsp butter in a large skillet over medium heat.

Sauté sliced onion, chopped mushrooms, chopped red bell pepper, and diced ham (if using) for 5 minutes, or until the onion is softened, stirring occasionally.

In a large bowl, whisk together the eggs, parmesan cheese, half of the chopped parsley, salt, and pepper.

Add the cooked onion mixture to the egg mixture and whisk to combine.

Cut the cooked spaghetti into 2-inch lengths.

Stir the spaghetti into the egg mixture.

Melt the remaining 1 tbsp of butter in the same skillet over medium heat, coating the sides and bottom.

Pour the egg mixture into the skillet and cook, without stirring, for 3 to 5 minutes, or until the bottom is golden and most of the mixture is set.

Carefully invert the frittata onto a plate.

Slide the frittata back into the skillet, cooked side up.

Cook for 3 to 5 minutes longer, or until golden on the bottom.

Garnish with the remaining parsley.

Serve warm or hot, cut into wedges.