Step 1

Cook onion and jalapeños over a gas burner, turning often, until charred and beginning to soften, about 4 minutes (alternatively, cook in a hot dry medium skillet, preferably cast iron). Let cool. Finely chop onion. Remove seeds from chiles and finely chop. Toss onion, chiles, cilantro, pumpkin seeds, oil, and lime juice in a small bowl. Season with salt.

Step 2

Do Ahead: Salsa can be made 1 day ahead. Cover and chill.

Step 3

Freeze pork until very firm around the edges, 20–30 minutes. Slice ¼" thick, then cut crosswise into 1½"–2" strips if needed. Cover and chill until ready to grill.

Step 4

Mix brown sugar, mustard powder, paprika, garlic powder, black pepper, and cayenne in a small bowl to combine.

Step 5

Prepare a grill for medium-high, indirect heat (for a charcoal grill, bank coals on one side of grill; for a gas grill, leave one or two burners off). Oil grates. Thread pork onto 6–8 skewers, folding and piling onto itself to form a compact shape. Season with salt, then sprinkle generously with brown sugar mixture in several passes, allowing a few minutes between each for rub to adhere. Grill over direct heat, turning every minute or so, until browned and beginning to char in spots, about 4 minutes. Move to cooler side of grill and continue to grill until cooked through, about 5 minutes longer. Serve pork topped with salsa.

Step 6

Do Ahead: Dry rub can be made 1 month ahead; store airtight at room temperature.