Combine the tequila, lime juice, onions, garlic, cilantro, salt, and pepper in a skillet over high heat and bring to a boil. Stir in the cream and simmer for 3 minutes.
Whisk in the butter and remove from the heat. Continue whisking until all of the butter is incorporated. Serve immediately, or prepare without the butter and refrigerate overnight in an airtight container. Reheat in a saucepan over a low flame. When the sauce is at a simmer, whisk in the butter and continue from there.
Yield: about 3/4 cup