Step 1

Place a rack in the highest position in oven; heat broiler. Toss red onion, lime juice, sugar, and a pinch of salt in a small bowl; set aside.

Step 2

Toss pineapple and 1 Tbsp. oil on a rimmed baking sheet and arrange in a single layer on half of baking sheet; reserve bowl. Broil pineapple until lightly charred around the edges, 5–8 minutes.

Step 3

Meanwhile, toss shrimp, Sriracha, and remaining 1 Tbsp. oil in reserved bowl until shrimp are evenly coated; season with salt and pepper and toss again.

Step 4

Remove baking sheet from broiler and use a spatula to turn pineapple pieces over. Arrange shrimp in a single layer on empty half of baking sheet. Broil until shrimp are cooked through on top, about 2 minutes. If your shrimp are opaque and springy to the touch, they're done. Remove from broiler and turn shrimp (leave pineapple alone). Broil again until shrimp are cooked through, 1–2 minutes. Transfer shrimp and pineapple to a large bowl and season with more salt and pepper.

Step 5

Drain soaking liquid from onion into bowl with shrimp and pineapple; set onion aside and toss shrimp and pineapple in pickling liquid.

Step 6

Serve shrimp mixture with tortillas, avocado, jalapeño, cilantro, lime wedges, and reserved pickled onion for making tacos.