In a bowl combine the grapefruit juice, jalapeño, shallot and half the avocado oil. Whisk until combined. Add the chicken, cover and refrigerate for 15 minutes.
Meanwhile heat a grill pan over medium heat. Brush the red and yellow peppers with the remaining avocado oil and place on the grill. Grill the peppers until tender and charred. Remove the stems, ribs and seeds. Slice and set aside.
Remove chicken from fridge. Place the marinated chicken on the grill, sprinkle with the salt and pepper and cover the pan loosely with aluminum foil. Cook until chicken can be lifted easily and has deep grill marks, 10 to 12 minutes.
Flip and continue to cook until the chicken is no longer pink inside, 10 to 12 minutes more. Transfer the chicken to a cutting board and slice.
Meanwhile, warm the tortillas in a pan or directly on a gas burner. Make a bed of lettuce on a tortilla and top with peppers and chicken. Garnish with jalapeño, cheese or fresh grapefruit salsa.