Combine the avocado, lime juice and cilantro, if using, in a bowl, and set aside.
Place the oil in a skillet over medium-high heat. Add the onion and bell pepper and cook until tender, about 3 to 4 minutes. Mix in the prawns, orange juice, cumin, Tabasco and salt. Cook until the prawns are heated through, about 2 to 3 minutes.
Divide and stuff the tacos with the lettuce, prawn mixture and cubed avocado.