Step 1Coarsely mash the avocado in a bowl with a fork. Stir in the mayonnaise, onion, lime juice and jalapeno, then salt and pepper to taste.
Step 2Spread the avocado mixture on the tortillas to the edges, about 2 tablespoons on each. Scatter about 12 cilantro leaves on each. Place 6 shrimp halves across the top 1/4 of the tortilla. Place the sliced tomatoes over the shrimp, dividing evenly. Start with the shrimp end and roll up tightly. Wrap airtight in plastic; twist ends. Refrigerate 2 to 6 hours.
Step 3Cut each in half (to accompany soup) or in 4 pieces (as hors d'oeuvre). Arrange on a platter; garnish with cilantro.