Look for ancho chile powder with the Mexican ingredients at your grocery store, on buy it online. (If you can't find ancho chili powder, you replace the ancho chili, the oregano, and the cumin with 2 1/2 tablespoons regular chili powder, though the flavor won't be quite the same.)
For the chicken:
- 2 tablespoons ancho chili powder
- 1 teaspoon dried oregano
- 1 teaspoon dried cumin
- 1 teaspoon sugar
- 1/2 teaspoon salt
- 1 clove garlic, finely chopped
- 1 tablespoon finely grated orange zest
- 3 tablespoons fresh-squeezed orange juice
- 2 tablespoons olive oil
- 4 to 6 skinless, boneless chicken thighs (1 1/4 pounds)
- 8 small corn tortillas
- 3 cups packed baby arugula (3 ounces)
- 2 medium ripe avocados, sliced
- 4 radishes, thinly sliced
- 1/2 pint cherry tomatoes, halved
- 1/4 red onion, thinly sliced
- Roughly chopped cilantro
- 1/2 cup sour cream thinned with 1/4 cup milk
- 1 lime, cut into wedges
1 Prepare a gas or charcoal grill for medium-high, direct heat.
2 Make the marinade and coat the chicken: In a large bowl, stir together the chili powder, oregano, cumin, sugar, salt, garlic and orange zest. Stir in the orange juice and olive oil to make a loose paste. Add the chicken to the bowl and toss to coat all over.
Set aside to marinate while the grill heats and you prepare the rest of the toppings.
3 Grill the chicken: Grill the chicken for 3 to 4 minutes per side, or until a thermometer inserted into the thickest part of the meat registers 165F. Transfer to a plate and rest for 5 minutes.
4 Warm the tortillas: Place each tortilla on the grill or on a hot, dry skillet over medium-high heat. As soon as you see pockets of the air start to puff up in the tortilla, turn it with tongs and heat for a few seconds on the other side.
Wrap warmed tortillas in a tea towel to keep them warm until serving.
5 Assemble the tacos: Slice the chicken into strips. On each tortilla, place a small handful of arugula. Top with chicken slices, sliced avocado, radishes, tomatoes, and onion slices. Drizzle with the thinned sour cream. Serve with lime wedges.