How to Make It
Heat 1/2 cup oil in a small frying pan over medium-low heat. Whisk in garlic, cumin, salt, oregano, and achiote paste (if using). Remove from heat and let stand 10 minutes.
Put garlic mixture, orange juice, and lime juice in a large resealable freezer bag. Add chicken, seal bag, and massage it to kick-start marination. Chill 8 hours.
Preheat a grill to medium (350° to 450°) with a large cast-iron skillet on one side of grill. Remove chicken from marinade; discard marinade. Grill chicken, covered, until an instant-read thermometer registers 165° when inserted into thickest portion of each piece, 20 to 30 minutes; turn often and move to a cooler area of grill as they reach temperature.
While chicken cooks, swirl remaining 1 tbsp. oil into hot skillet on grill. Add onion slices and jalapeños and cook, in batches if necessary, until onions are golden brown and tender, about 5 minutes per side, and jalapeños are blistered on all sides, about 8 minutes. Serve onions and jalapeños with chicken.