1. Warm oil in a large skillet over medium-high heat. Add onion and cook, stirring occasionally, until softened, about 3 minutes. Add peppers and sauté until softened, 5 to 7 minutes. Add pepperoni and cook, stirring, for 1 minute.
2. While pepper mixture is cooking, lightly toast bread slices in toaster oven.
3. Stir arugula into pepper mixture until it just begins to wilt slightly. Pile pepper mixture on top of bread slices and serve.