Cut the brioche rolls in half. Remove a little of the soft bread from the base and tops to form a cavity. Set 18 tomatoes aside. Cut the rest crossways each into four rounds. Place into a large bowl. Add the prawns, avocado, lime juice and season.
Combine the mayonnaise and sriracha, spoon half a cup over the tomato mixture and stir gently to coat.
Preheat oven 160°C fan forced. Warm the roll bases and tops for 5 minutes. Spoon the tomato and prawn mixture into the base of the warm rolls. Top with watercress and brioche lid. Press a skewer into each reserved tomato and insert into the sliders to secure. Serve with remaining mayonnaise.