Combine 1/2 lime juice, 1/2 cup cilantro, 1/2 cup olive oil, soy sauce, tequila, 7 cloves garlic, lime zest, cumin, oregano and pepper in a medium casserole dish.
Place the tri-tip in the marinade to cover both sides.
Marinate the tri-tip for at least 4 hours (preferably over night), flipping occasionally.
Grill the tri-tip over medium-high heat until cooked to desired temperature (about 8 minutes per side for medium rare). Cover and allow to rest for 10 minutes.
For the sauce, combine the sour cream, avocado and remaining ingredients and whip with a fork.
Top steak with sauce to serve.