Cook the fettuccine according to package directions, omitting added oil and salt. Drain the pasta, reserving 1/4 cup cooking liquid.
Meanwhile, heat the oil in an extra-large nonstick skillet over medium heat. Sauté the chicken, onion, and jalapeño for 10 minutes or until the chicken is done. Add the tequila and 2 tablespoons lime juice. Sauté for 5 minutes or until the liquid has evaporated, yet the mixture is still moist. Stir in 1/2 teaspoon garlic salt.
Add the pasta to the chicken mixture and toss, using long-handled tongs. Add the cheese, reserved cooking water, evaporated milk, remaining lime juice, and remaining garlic salt. Toss to combine well. Remove from heat once the liquid is absorbed, top with cilantro, and serve immediately.