Whisk together first 7 ingredients until well blended.
Place onion and bell pepper in 1 zip-top bag; add 1/2 cup beer mixture. Seal, turning to coat. Chill 1 hour.
Place chicken in remaining zip-top bag; add remaining beer mixture. Seal, turning to coat. Chill 1 hour, turning once.
Drain chicken, discarding marinade.
Grill chicken and onion, covered with grill lid, over high heat (400° to 500°) 5 to 6 minutes on each side. Grill bell pepper 5 minutes.
Cut chicken into thin slices, and cut bell pepper into thin strips. Serve chicken, onion, and bell pepper with flour tortillas. Serve with desired toppings.
*1 (1 1/2-pound) package flank steak may be substituted for chicken. Marinate 4 hours. Grill flank steak 10 minutes on each side.