Prepare the marinade: in a bowl, combine olive oil, lime zest and juice, cumin, smoked paprika, oregano, chipotle powder (if using), minced garlic, salt, and pepper. Mix until well blended.
Slice the chicken thighs into even strips or small chunks to maximize surface area for quick browning.
Toss the chicken in the marinade until thoroughly coated. Let it rest at room temperature for about 10 minutes, or refrigerate for up to 2 hours for deeper flavor.
Heat a heavy skillet or grill pan over medium-high heat. Sear the chicken pieces in a single layer, working in batches if needed, until nicely browned and cooked through, about 6–8 minutes per batch depending on thickness.
Warm the tortillas: either wrap in a clean kitchen towel and microwave for 20–30 seconds, or warm in a dry skillet for about 15–20 seconds per side until pliable.
Assemble: lay a tortilla on a plate, add a generous scoop of chicken, then top with red onion, cilantro, jalapeño, radishes, and pineapple if using. Finish with cotija cheese and a spoonful of mango or pico de gallo salsa.
Finish with a squeeze of lime and a light pinch of salt, if desired, to sharpen the flavors.
Serve immediately while warm and juicy. These tacos pair beautifully with a simple cucumber-lime agua fresca or a chilled lime beer for a festive atmosphere.