* Shrimp can be either fresh or frozen and should be shelled.

** Jalapeno Pepper should be rinsed, seeded and cut into strips.

Thaw shrimp if frozen. In a bowl combine vinegar, lemon juice, salt, mustard and pepper and set aside. In a medium skillet, cook the shrimp, half of the onion slices and the garlic in hot oil over medium-high heat for 4 to 5 minutes, or just until shrimp are done, stirring occasionally. Remove onion and garlic with a slotted spoon and discard. Add shrimp and the remaining oil to the vinegar mixture in the bowl, along with remaining onion slices and the jalapeno pepper. Cover and chill for several hours or overnight, stirring occasionally. To Serve: Lift the shrimp, onion slices and jalapeno pepper from the marinade and spoon into the avocado halves. Sprinkle with the chopped tomato and drizzle some of the remaining marinade over all.