In a large bowl, combine the vinegar, lemon juice, salt, mustard and pepper; set aside. In large skillet over medium-high heat, cook and stir shrimp, half the onion slices and garlic in 2 tablespoons oil for 4-5 minutes or until shrimp turn pink.
Remove onion and garlic with slotted spoon; discard. Add shrimp and remaining oil to vinegar mixture, along with jalapeno pepper and remaining sliced onion. Cover; stir occasionally. Chill overnight.
To serve, spoon shrimp mixture into avocado halves. Sprinkle with tomato; drizzle with some of marinade. Yield: 4 servings.