1. Place the shrimp in a large pot of boiling, salted water. Remove the pot from the heat, cover and let stand until the shrimp are opaque and cooked through, about 3 minutes. Drain the shrimp, pat dry and refrigerate.
2. Preheat a grill pan over medium-high heat. Lightly brush the pineapple slices, onion slices and jalapeño with oil on both sides and cook, turning once, until grill marks appear, about 3 minutes on each side.
3. Coarsely chop the onion and pineapple. Seed and chop the jalapeño. In a large bowl, toss the onion, pineapple and jalapeño with the mint and lime juice; season to taste with salt.
4. Add the reserved shrimp and toss.
5. To serve, spoon the shrimp salad onto the tostada shells.

Serves 4.