Step 1

In a bowl, combine the butter, tequila, lemon and lime juice, thyme, serrano, garlic, cilantro, and onion and mix well.

Step 2

Place steak in a large sealable bag and pour the marinade over steak. Zip up the bag and massage marinade around so that the steak is well covered. You can also use a shallow dish if you prefer. Let stand at room temperature for 30 minutes or in the refrigerator for up to 2 hours.

Step 3

Preheat a grill or grill pan to high. Remove the steak from the plastic bag and generously season with salt. Grill the steak for 5 to 7 minutes per side for medium-rare. Let rest 10 minutes, and then slice the steak against the grain into 1/2-inch pieces.

Step 4

Place desired amount of meat into a warm corn tortilla. Top with salsa, avocado, and other toppings as desired. Garnish with a wedge of lime, to be squeezed over taco before eating.