Heat a large pan over medium high heat and when hot, add the oil. When the oil is shimmering, add the onion and cook until it is softened. Add bell peppers and continue to cook, stirring often until peppers and onion brown.

Add sliced kielbasa and cook till warmed. Add vinegar, soy, monk fruit, and pepper flakes, if using. Cook until the onions and bell peppers are blistered, the sausage is browned, and liquid is almost completely evaporated. Add black pepper, taste and adjust the vinegar, sweetener, and salt seasonings.