Serve this alongside our Cocktail Franks in a Baguette.
Heat the oil in a medium skillet over medium-high heat. Add onions; cook, stirring occasionally, until beginning to soften, about 7 minutes. Reduce heat to medium. Add tomatoes; cook, stirring occasionally, 10 minutes.
Stir in sugar, vinegar, salt, and cayenne pepper. Cook, stirring occasionally, until onions are soft and mixture is shiny, about 20 minutes (if the onions begin to brown, reduce heat to medium-low). Serve warm or at room temperature.
Relish can be refrigerated in an airtight container up to 3 days.