Heat oil in a large saucepan over medium-high heat. Add onion and next 3 ingredients; cook, stirring often until tender. Stir in broth and, if desired, tequila. Bring to a boil; reduce heat, and simmer 10 minutes. Remove from heat, and cool 20 minutes.
Process tomatillo mixture in a blender until smooth, stopping to scrape down sides. Add remaining ingredients; process until blended.