Place soy sauce, lime juice, canola oil, brown sugar, cilantro, garlic, chili powder, jalapeño pepper, cumin seed, and black pepper in the bowl of a blender; process until finely chopped. Pour into a gallon-sized zip-top bag; add the skirt steak and seal, removing as much air as possible. Refrigerate for 2 to 12 hours, turning from time to time to distribute marinade.

Toss peppers and onion with a little olive oil and lime juice; grill 8 to 10 minutes or until just beginning to go limp. Set aside and keep warm. Heat a grill to medium-high. Remove skirt steak from marinade about 30 minutes before you intend to put it on the grill; sprinkle generously with salt and pepper and allow to come to room temperature. Lightly oil grill; add skirt steak and grill 4 minutes per side or until desired degree of doneness. Remove to a cutting board to rest for 5 minutes. 

Grill tortillas, turning once, until warm and beginning to color. Thinly slice steak across the grain and allow guests to assemble fajitas as desired.