Put the chicken, minced garlic, olive oil, 1 tablespoon lime juice (if using), chili powder, paprika (if using), garlic or onion powder, 1/2 teaspoon salt, and 1/4 teaspoon black pepper into a large bowl or a zip-top bag; mix or shake to coat the chicken evenly.
Heat a large skillet over medium-high heat until hot. Add the chicken and cook 6–7 minutes per side, or until cooked through and an instant-read thermometer registers 165°F (74°C).
Remove the chicken from the pan and let rest at least 5 minutes, then slice or chop into small cubes.
While the chicken cooks, combine the finely chopped tomato, jalapeño, onion, cilantro, 1 teaspoon lime juice, and a pinch of salt and pepper in a small bowl; stir to make the topping.
Warm or lightly char the tortillas over the stovetop flame or in a dry skillet until pliable and slightly charred, if desired.
Assemble the tacos by placing about 1/4 cup of the chopped chicken into each tortilla and topping with a few tablespoons of the onion–tomato mixture. Serve with cilantro sauce if desired.