In a sauté pan over medium-high heat, warm 2 Tbs. olive oil. Add the red and yellow onions and sauté, stirring occasionally, until tender and golden, 7 to 10 minutes. Add the garlic and sauté, stirring occasionally, until fragrant, 1 to 2 minutes more. Add the vinegar and cook, stirring often, until the liquid is nearly evaporated, 3 to 5 minutes more. Season with salt and pepper, remove from the heat and let cool to room temperature. Cover and refrigerate until ready to serve.