Mix 2 cups mayonnaise, 2 tbsp Thai-style sweet chili sauce, and 2 tbsp Sriracha chili sauce together well. Set aside as Sriracha Mayonnaise.
Mix 6oz. red onion thinly sliced, 12oz. thinly sliced mango, 1 cup Thai basil leaves (torn in half), 1/2 cup lime juice, 4 tbsp vegetable oil, and 3 tbsp salt all together in a separate bowl. Set aside as Mango Salad.
Bake bread according to package directions. Cool, then split in half and grill lightly. Spread both sides of bun with Sriracha mayonnaise. Mix heated and shredded beef with remaining 1 tbsp of Thai-style sweet chili sauce.
Spoon Thai beef on bottom half of Ciabatta. Top with Mango Salad, then cover with top bun.