NOTES: For best texture, don't marinate the chicken longer than 4 hours. To have guests grill their own fajitas, place raw chicken and vegetables on separate plates in step 3. Set cutting boards near the grill along with knives for slicing cooked chicken (step 5). If there's not enough grill space for everything, cook vegetables first and serve at room temperature.
1 cup orange juice
1/2 cup lime juice
1/2 cup tequila
2 tablespoons olive oil
1 1/2 teaspoons chipotle chile purée (see below)
1 teaspoon salt
3 pounds boned, skinned chicken breasts, rinsed and patted dry
In a large bowl, mix orange juice, lime juice, tequila, oil, chipotle purée, and salt. Reserve 1/2 cup of the mixture. Place each chicken breast half between 2 sheets of waxed paper or plastic wrap; with a flat mallet or a rolling pin, pound until breast is an even 3/4 inch thick. Peel off waxed paper or plastic.
Place chicken in bowl with marinade; turn to coat. Cover and chill at least 30 minutes or up to 4 hours (see notes).
Slice onions 3/4 inch thick. Rinse, stem, and seed peppers; cut into eighths lengthwise. Brush vegetables generously with reserved orange-juice mixture.
Set vegetables on a barbecue grill over a solid bed of medium coals or medium-high heat on a gas grill (you can hold your hand at grill level only 4 to 5 seconds); close lid on gas grill. Cook, turning as needed, until vegetables are browned and tender when pierced, 8 to 12 minutes total. Transfer to a platter as cooked.
Lift chicken from marinade (discard used marinade) and lay on grill; close lid on gas grill. Cook, turning chicken as needed to brown evenly, until no longer pink in the center (cut to test), 6 to 10 minutes total. Transfer to a board and let stand until cool enough to handle, then slice across the grain 1/2 inch thick.
To assemble fajitas, lay tortillas on grill as needed, turning quickly, just until warm, 3 minutes total. Fold a warm tortilla around chicken, peppers, and onion; top with avocado-orange salsa.
To make chipotle chile purée, whirl the contents of a 7-ounce can of chipotle chiles (including sauce) in a blender or food processor until smooth. Scrape into a plastic container and store airtight in the refrigerator for up to 3 weeks.