1. Preheat oven to 425 degrees. In a 10-inch ovenproof nonstick skillet, heat oil over medium-high. Add onion and spinach, season with salt and pepper, and cook until onion is translucent, about 5 minutes.

2. Add eggs and bacon, season with salt and pepper, and stir to combine. Cook, undisturbed, until edges are set, about 2 minutes. Transfer skillet to oven and bake until top of frittata is just set, 10 to 13 minutes. Invert or slide frittata onto a plate and cut into 6 wedges. Serve warm or at room temperature.