In a blender or food processor combine first 5 ingredients and blend until smooth. In a shallow nonreactive pan pour blender mixture over chicken breast half and set aside to marinate, covered and refrigerated, 4 hours. Preheat a grill or broiler. Grill or broil chicken and onions 2 to 3 minutes on each side, until chicken is just cooked through and onions are browned. Thinly slice chicken.
Preheat oven to 400 degrees and butter an 8-inch round cake pan. In pan, layer one tortilla and sprinkle with half of chicken, onion, cheeses and basil. Top with a tortilla and sprinkle with remaining filling ingredients. Top with third tortilla and brush top with butter. Bake for 15 minutes, or until lightly browned and cheeses have melted. Let rest 5 minutes before cutting into quarters. Serve garnished sour cream and cilantro.
Recipe courtesy of David Rosengarten