Combine onion, bell pepper, garlic and ginger in a mini food processor. Pulse, scraping down the sides as necessary, until very finely chopped. Heat 1 tablespoon peanut (or canola) oil in a small skillet over medium heat. Add the onion mixture and cook, stirring occasionally, until softened, about 5 minutes. Transfer to a large heatproof bowl and spread into a thin layer. Let cool for 5 minutes.