Preheat the broiler. Arrange the peppers skin-side up on a baking sheet and broil until charred, about 3 minutes. Transfer to a bowl and cover with plastic wrap; set aside. Preheat the oven to 400 degrees F. Soak the onion in a small bowl of cold water; set aside.
Whisk the eggs and egg whites in a medium bowl with 1/4 cup water and a pinch each of salt and pepper. Heat 1 tablespoon olive oil in a 10-inch nonstick ovenproof skillet over medium-high heat. Add the garlic and mint; cook until sizzling, 20 seconds. Add the spinach and cook, stirring, until wilted, 2 minutes; season with salt and pepper. Stir in the eggs using a wooden spoon; top with the feta. Cook, undisturbed, until the mixture starts setting around the edges, 2 minutes. Bake in the oven until set and lightly golden, 15 minutes.
Meanwhile, remove the charred skins from the peppers; slice into strips. Drain the onion and toss with the peppers, olives, vinegar and the remaining 1 tablespoon olive oil. Season with salt and pepper.
Slide the frittata onto a board; let rest 5 minutes. Slice into wedges; drizzle with olive oil. Serve with the pita bread and salad.