Place prickly pear in a fine-mesh strainer set over a bowl. Mash with a potato masher to extract juice. Discard seeds and pulp.

Preheat oven to 400 degrees F (200 degrees C).

Heat oil in a large skillet over medium heat. Add chicken; cook and stir until juices run clear, 5 to 8 minutes.

Divide cooked chicken among 6 tortillas. Top with mozzarella cheese. Fold in opposing ends and roll up tortillas. Place seam-side down in a casserole dish.

Bake in the preheated oven until cheese is melted and tortilla edges are crisp, about 15 minutes.

Melt butter in a saucepan over medium-low heat. Add onion, bell pepper, garlic, and salt; cook and stir until very tender, about 5 minutes. Pour in prickly pear juice, water, lemon juice, bouillon cube, cornstarch, and pepper sauce; whisk until cornstarch dissolves into the sauce.

Bring sauce to a simmer over medium heat. Stir in sour cream. Simmer until flavors combine, about 3 minutes. Remove from heat; stir in parsley. Spoon sauce over tortillas.