How to Make It
Whisk together mayonnaise, jelly, relish, onion, lemon juice, garlic powder, and 1/4 teaspoon salt.
Using your hands, gently mix buffalo, brisket, and Worcestershire sauce in a medium bowl just until combined. Gently divide mixture into 16 portions. Sprinkle evenly with black pepper and 1 teaspoon salt.
Heat a large cast-iron skillet over high. Coat hot pan with cooking spray. Add 8 meat portions to pan. Using a flat heatproof spatula, press each portion into a 4-inch patty (it's OK if the edges are a little craggy). Cook until burger edges are very crispy and patties are cooked through, 3 to 4 minutes per side. Transfer to a plate; cover loosely with aluminum foil to keep warm. Repeat procedure with remaining 8 meat portions.
Wipe skillet with a paper towel. Add 1 teaspoon oil to pan; heat over medium-high. Add radicchio; cook until lightly charred and crisp-tender, 2 to 3 minutes per side. Transfer to a cutting board, and roughly chop. Toss together radicchio, vinegar, remaining 1 teaspoon oil, and remaining 1/4 teaspoon salt in a bowl.
Spread about 1 teaspoon mayonnaise mixture on bottom halves of buns. Top evenly with meat patties and radicchio. Cover with top halves of buns.