Heat broiler. Heat the oil in a large skillet over medium heat. Add the onion, 1/2 teaspoon salt, and 1/4 teaspoon pepper and cook, stirring occasionally, until the onion begins to soften, 5 to 6 minutes. Add the bell pepper and cook, stirring occasionally, until softened, 5 to 6 minutes. Add the spinach and cook, tossing, until the spinach is just wilted, 1 to 2 minutes.
Meanwhile, remove the sausages from their casings and form into twelve 1/2-inch-thick patties. Place them on a broilerproof rimmed baking sheet and broil until cooked through, 4 to 5 minutes per side.
Form sandwiches with the rolls, sausage patties, and vegetable mixture. great for guests
These bite-size sandwiches make easy hors d'oeuvres. Cook the vegetables and form the patties up to a day in advance; broil the patties, reheat the vegetables, and assemble the sliders just before guests arrive.