To make the marinade: In a small, heavy saucepan over medium heat, warm the oil and cook the garlic until lightly browned, 4 to 5 minutes, turning occasionally. Add the onion and chiles. Cook for about 3 minutes, stirring occasionally. Add the remaining marinade ingredients, bring to a boil, then simmer for lf minutes. Remove the saucepan from the heat and let the mixture stand for 15 minutes to soften the chiles and blend the flavors. Purée the marinade in a blender.