Finely chop the 1/2 onion + chili pepper. Crush the garlic.
Put oil in saucepan, heat. Add onion, fry untio golden. Add garlic and chili and fry until fragrant.
Remove from heat, add peanut butter and stir smooth.
Back on heat, stir in water, lemon juice, balsamic vinegar and sugar. Thicken for 3 minutes or so.