Place a dry skillet over medium heat and warm the corn tortillas for about 30 seconds per side until they develop slight blisters and are soft. Keep them warm covered with a cloth.
Add a tablespoon of oil to the same skillet, then toss in the sliced onions. Sauté on medium-low heat, stirring often, until they turn golden brown and sweet, about 10 minutes. The onions should smell fragrant and caramelized.
While the onions cook, chop or shred your leftover cooked chicken into bite-sized pieces. If raw, cook the chicken in a separate pan with a pinch of salt and spices until heated through and slightly charred for extra smoky flavor.
Once the onions are ready, remove them from the heat and set aside. Assemble your tacos by placing a warm tortilla on a plate, then layering with chicken, caramelized onions, and a spoonful of spicy salsa.
Sprinkle crumbled queso fresco over the top, then garnish with freshly chopped cilantro. Squeeze lime juice over everything for a bright, tangy finish.
If you like, toss the assembled tacos briefly in the hot skillet for about 30 seconds to melt the cheese slightly and crisp up the tortillas. This also helps meld the flavors and adds a crispy edge.
Remove the tacos from the skillet, let them rest for a minute, then serve immediately. Enjoy the crispy, smoky, spicy bites that are packed with flavor and perfect for a quick, satisfying meal.