2 tsp olive oil
1/2 cup chopped red onion
1 1/2 cup sliced mushrooms
1 red pepper, roasted and chopped
2 T balsamic vinegar
1/2 bunch spinach
1/4 cup plus 2 T chopped basil, divided
salt & pepper
1/2 cup parmesan cheese
1 cup crumbled feta cheese
3/4 cup chopped tomatoes, drained
Heat oil in nonstick ovenproof skillet over medium heat. Add the onion and saute until tender, about 8 minutes. Add mushrooms and cook until they release their liquid and begin to brown, about 5-7 minutes. Stir in the chopped red pepper, followed by the vinegar, stirring to scrape brown bits from pan. Cook until vinegar evaporates, about 2 minutes. Add spinach and stir until fully wilted, 2-3 minutes. Stir in 1/4 cup basil with salt and pepper to taste.
In a medium bowl, whisk together eggs, parmesan, 1/2 tsp salt, and 1/4 tsp pepper. Pour the eggs over the hot veggies in the skillet and gently stir for a few seconds to combine. Reduce the heat to med-low and cook without further stirring until the eggs are set on the bottom, 5-8 minutes. Preheat broiler.
Sprinkle the feta cheese over the frittata and broil until cheese melts and begins to brown, 2-4 minutes. Remove from oven, sprinkle with tomatoes and remaining 2 T basil and serve.