In a large skillet, heat 1 tablespoon oil over medium-high. Add onions and peppers; cook, stirring occasionally, until beginning to brown, about 7 minutes. Season with salt and pepper and add 3/4 cup water; cover and simmer 3 minutes. Uncover and cook, stirring occasionally, until vegetables are soft and liquid has evaporated, 2 minutes. Cover and refrigerate 1 cup onion-pepper mixture for later use.
Meanwhile, heat 1 teaspoon oil in a small skillet over medium. Add sausages and cook, turning occasionally, until browned and cooked through, 10 to 12 minutes. Place sausages and vegetables on bread and top with mustard if desired.