Preheat the oven to 375 degrees F. Heat the olive oil in a 12-inch oven-safe nonstick skillet set over medium heat. Add the onions and cook, stirring occasionally, until translucent and lightly golden, 7 to 10 minutes. Whisk the milk, salt and eggs together in medium bowl. Add the egg mixture to the skillet, then add the peas. Add the black pepper and cook the frittata until the edges begin to set and turn pale yellow, 5 to 7 minutes. Place the salmon pieces in a single layer on the eggs. Place the skillet in the middle of the oven and bake until the center is set and no longer jiggles, 8 to 10 minutes. Remove from the oven and cool for 5 minutes. Cut the frittata into six wedges and garnish with the chopped parsley.
If your skillet is larger or smaller than 12 inches, adjust the cooking time accordingly. Larger skillets will require less cooking time and the frittata will be flatter.