Heat oil in medium saucepan over medium heat until shimmering. Add onion and cook until softened, about 7 minutes. Add peppers, vinegar, sugar, garlic, ginger, mustard, salt, and pepper flakes. Bring to simmer and cook until thickened, about 40 minutes. Cool to room temperature, about 2 hours or overnight. Serve with pork.

Makes about 2 cups