Rinse peppers under cold water and pat dry. Slice off tops and bottoms, stand upright, and cut down along the inside to remove seeds and white ribs. Slice lengthwise into ½-inch strips; halve crosswise for shorter pieces. Keep sizes uniform so everything cooks evenly. If adding onion, slice it now. Measure out olive oil, salt, and pepper. Have everything ready before the pan goes on — sautéing moves fast.

Heat a large Skillet over medium heat. Add 1 tbsp olive oil per 2 peppers; warm 30–60 seconds until shimmering. Add pepper strips in a single layer — work in batches if crowded. Season with salt and pepper, stir to coat. Cook over medium heat, stirring every 2–3 minutes, for 6–10 minutes. At 6 minutes: slightly softened with snap. At 10 minutes: tender throughout with light color. For very soft peppers, cook 30–40 minutes over medium-low, stirring occasionally. Add garlic halfway through cooking. If browning too fast, add a splash of white wine or water and briefly cover to steam.

Heat the 2 tbsp extra-virgin olive oil in a Skillet over medium heat. Add the 1 small yellow onion thinly sliced and 1 whole red bell pepper diced and cook, stirring occasionally, until softened, about 5–6 minutes. Add the 2 cloves garlic minced and cook for 30 seconds until fragrant. Add the 2 cups baby spinach loosely packed and stir until just wilted, about 1 minute.

Spread the vegetables in an even layer across the bottom of the Skillet. Pour the egg mixture evenly over the top. Cook undisturbed over medium-low heat until the edges are set and the center is still slightly jiggly, about 5–6 minutes.

Scatter the remaining ¾ cup shredded cheddar cheese divided over the top of the frittata. Transfer the Skillet to the Broiler and broil until the top is golden, puffed, and fully set in the center, about 2–3 minutes. Watch closely — broilers vary. Remove from the Oven and let rest for 2 minutes.