How to Make It
In a small saucepan, bring the vinegar, sugar and a pinch of salt to a boil, stirring to dissolve the sugar. Pour the hot pickling liquid over the red onion in a small, heatproof bowl and let cool, then refrigerate overnight.
In another saucepan, combine the orange juice, white onion, maple syrup, malt vinegar and cumin seeds. Boil over moderately high heat until the liquid has reduced to 2 tablespoons, about 3 minutes. Transfer to a blender, add the oil and lime juice and puree until smooth; season with salt.
Drain the red onion and transfer to a bowl. Add the mango, cabbage, cilantro and half of the vinaigrette and season with salt.
In a skillet, heat the oil. Add the onion and cook over moderate heat until softened, 5 minutes. Add the shrimp, season with salt and pepper and cook, turning, until white throughout, 3 minutes. Add the barbecue sauce and bring to a simmer. Off the heat, stir in the cheese.
Spoon the shrimp and red onion salad onto the tortillas and roll them up. Garnish with the pumpkin seeds, chile and cilantro, and serve with the remaining vinaigrette.