In a large microwave-safe bowl, add the sliced onions and bell peppers. Cook on high in microwave for 8-10 minutes, until soft.
In the meantime, cut the 4 sausages into thin slices, about ¼ inch thick. Coat a large, nonstick pan with cooking spray. Add the sausage slices and cook until slightly browned, about 5 minutes. Stir often. Remove sausages to a plate.
In the same pan, coat again with cooking spray. Add 2 teaspoons olive oil. Add the cooked onions and bell peppers, oregano, basil, a little salt, and pepper. Cook over medium-high heat, stirring often, until the vegetables are soft and slightly caramelized, about 10 minutes. If the pan gets too dry, add 1 more teaspoon of olive oil and 2 tablespoons of water. Add the garlic and cook, stirring, until fragrant and soft, 1 minute. Return the sausages to the pan. Add 1 tablespoon balsamic vinegar. Mix everything together and continue to cook, stirring, until warmed through and well blended, about 2 minutes.
Arrange the sausages, peppers, and onions on a platter.
Serve with mustard on the side and toasted bread or rolls, if desired.
Makes 4 main course servings. Each serving 1¼ cup.Makes 6-8 appetizer servings