Finely grate the zest from 1 lime and put it in a 9x13-inch baking dish. Finely grate 1 teaspoon zest from the second lime and set aside for the salsa. Juice the limes. Add 1/4 cup juice to the zest in the baking dish and mix 1 teaspoon juice into the zest for the salsa.

To the baking dish, add the tequila, oil, cilantro, Cointreau, garlic, and pepper flakes; whisk to combine. Season the steak all over with 1/2 teaspoon salt. Add it to the marinade and turn to coat. Cover with plastic wrap and refrigerate for 2 hours, turning the steak after 1 hour.

Prepare a medium gas or charcoal grill fire. Scrub the grill grate with a wire brush and then wipe clean with a paper towel dipped in oil.

Rub the tomatillos and yellow pepper all over with the 1 tablespoon oil and season with 1/2 teaspoon salt. Grill the tomatillos, turning occasionally, until they have good grill marks and are starting to collapse, about 6 minutes. Grill the pepper, turning occasionally, until charred all over, 8 to 10 minutes. Transfer the tomatillos to a plate and let cool. Put the pepper in a small bowl and cover with plastic wrap. Let sit until cool enough to handle.

Roughly chop the tomatillos and put them in a medium bowl. Peel the skin from the pepper, remove the seeds, and cut into small dice. Add the pepper to the tomatillos along with the avocado, onion, cilantro, jalapeno, and the reserved lime zest and juice. Season to taste with salt and pepper.

Remove the steak from the marinade and pat it dry. Clean and oil the grill grates again. Grill the steak, covered, over medium heat until brown grill marks form on one side, about 5 minutes. Flip and cook until medium rare (130 degrees F on an instant-read thermometer), 4 to 5 minutes more. Transfer to a cutting board and let rest for 5 minutes. Cut each steak crosswise into 2 or 3 pieces, and then thinly slice across the grain. Serve with the salsa on the side.

The salsa may be prepared up to 2 hours ahead.