1. Warm oil over medium heat in a 12-inch nonstick skillet with an ovenproof handle. Add onion and bell pepper; cook, stirring often, until softened, about 5 minutes. Add spinach and salmon; season with salt and pepper. Sauté until heated through.
2. Preheat broiler to high and set an oven rack about 5 inches from heat source. Pour eggs into skillet and stir briefly to distribute vegetables and salmon. Reduce heat to medium-low and cook, sliding a spatula under frittata occasionally to loosen, until set underneath but still wet on top, about 8 minutes.
3. Place skillet under broiler; cook until top is golden, 1 to 2 minutes. Slide frittata onto a platter and cut into wedges.