In a large bowl, combine ground chuck and taco seasoning mix. Form mixture into 4 equal-sized patties.
Preheat grill to medium-high heat (350 to 400 degrees F). Spray grill rack with nonstick, nonflammable cooking spray.
Grill burgers, covered with grill lid, for 5 minutes per side, or until desired degree of doneness.
Top each patty with 1 slice cheese and grill, covered with grill lid, until cheese is melted (about 1 to 2 minutes).
Spread butter evenly on both sides of toast. Grill toast for 1 to 2 minutes per side, or until golden brown.
In a small bowl, combine salsa with taco sauce. Set aside. Spread 2 tablespoons sour cream on 4 toast slices.
Top each with 1 cup fried onions, 1 burger, and 2 tablespoons salsa mixture. Cover with remaining 4 toast slices.