Serve this tangy jam with TV chef Nora Singley's Chicken Liver Mousse recipe.
Heat olive oil in a large skillet over medium heat. Add onion, thyme, and sugar; season with salt and pepper. Cook, stirring, until onions are soft and lightly golden, about 15 minutes. Add vinegar and raisins; increase heat to high. Cook until liquid is syrupy and nearly evaporated, about 5 minutes. Remove thyme and discard. Season with salt and pepper. Remove from heat and let cool to room temperature before serving. Jam can be stored in an airtight container, refrigerated, for up to 3 weeks.