Preheat the oven to 375°F (190°C).
Season the chicken thighs on both sides with kosher salt and black pepper.
Heat 1 tablespoon olive oil in a large oven-safe skillet over medium-high heat until shimmering, then add the chicken and brown about 2 minutes per side.
Cover the skillet and transfer it to the preheated oven; roast until the chicken reaches 165°F (74°C), about 15–20 minutes.
While the chicken cooks, prepare the spice blend by combining 2 teaspoons ground cumin, 1 teaspoon smoked paprika, 1/2 teaspoon chile powder, 1/2 teaspoon garlic powder, and 1/2 teaspoon onion powder in a small bowl.
Make the quick pico: combine quartered grape tomatoes, finely diced red onion, chopped cilantro, and the juice of 1 lime; season with a pinch of salt and pepper to taste.
When the chicken is done, transfer it (with any pan juices) to a mixing bowl or the bowl of an electric mixer.
Using an electric mixer on low speed (or two forks), shred the chicken briefly, then add the prepared spice blend and continue mixing on low for 1–2 minutes until the chicken is fully shredded and spices are evenly incorporated.
Return the shredded, seasoned chicken and any accumulated juices to the skillet and toss to combine so the chicken stays moist.
To assemble each taco, heat a clean nonstick skillet over medium-low. Sprinkle about 1/2 ounce (roughly 1 tablespoon) grated cheddar into the skillet and let it begin to melt and crisp.
Place one 4-inch flour tortilla directly on top of the melting cheese, then add another 1/2 ounce of cheddar on top of the tortilla.
Spoon a portion of the shredded chicken onto the tortilla, top with a spoonful of the pico, and let the bottom cheese turn golden and crispy.
Gently fold the tortilla in half and transfer the taco to a plate. Repeat with remaining tortillas, cheese, and filling.
Serve immediately with extra pico and an optional squeeze of lime.
