Preheat oven to 400 degrees F.
Heat the extra-virgin olive oil in a small 6 to 8-inch oven-proof skillet over medium heat. Add onion to the hot oil and cook until soft and tender but not caramelized - keep the onion white. Add roasted peppers and olives and combine. Beat the eggs with half-and-half or milk and season them with salt and pepper. (Go easy on the salt because of the olives.) Pour the eggs into the skillet and keep settling them to the bottom of the pan as you would an omelet. When the eggs are firm on the bottom and set, transfer pan to the oven for about 10 minutes until top is golden brown. Cut and serve from the skillet or invert the frittata on to a serving plate.