This healthy take on chicken tostadas is delicious and easy to prepare.
Preheat oven to 375 degrees, with rack in center. Between two sheets of waxed paper, pound chicken breasts with a meat pounder until about 1/2 inch thick. Transfer chicken to a resealable plastic bag. Add lime juice, oil, salt, and pepper; refrigerate.
Place tortillas on a baking sheet. Brush tops with oil, and season with salt and pepper. Bake until crisp and golden, 10 to 12 minutes. Remove from oven; set aside to cool.
Heat a grill or grill pan over medium-high heat. Remove the chicken from refrigerator; discard marinade. Grill chicken until browned and cooked through, about 5 minutes per side. Let rest until cool enough to handle, about 5 minutes; cut into strips.
Brush onions with oil; grill until browned, about 3 minutes per side. Serve the tortillas topped with lettuce, tomatoes, avocados, chicken, onions, cilantro, and sour cream.