1. Preheat the oven to 450 degrees F. Place the bell pepper directly on a stovetop gas burner over high heat and char on all sides, turning with tongs, about 5 minutes. (If you don’t have a gas stove, do this on a baking sheet under the broiler.) Transfer the pepper to a medium bowl, cover tightly with plastic wrap and let sit 10 minutes.
2. Meanwhile, heat 2 tablespoons olive oil in a medium nonstick ovenproof skillet over medium-high heat.
3. Add the onions and spinach and cook, stirring, until wilted, about 4 minutes.
4. Stir in 1/2 teaspoon salt, and pepper to taste; remove from the heat.
5. Whisk the eggs, 2 tablespoons breadcrumbs, 3/4 cup water and 1/2 teaspoon salt in a large bowl.
6. Add the egg mixture and cheese to the skillet and stir to combine.
7. Sprinkle with the remaining 2 tablespoons breadcrumbs.
8. Transfer the skillet to the oven and bake until the frittata is set and the top is golden, about 15 minutes.
9. While the frittata is cooking, peel off the skin from the roasted red pepper with your fingers.
10. Cut the pepper in half, discard the seeds and slice into strips. Toss with the remaining 2 tablespoons olive oil, and salt and pepper to taste in a bowl.
11. Slide the frittata onto a serving plate and cut into wedges.
12. Serve with the roasted pepper.