Preheat one side of your grill to 400°F, leaving the other side off for indirect grilling.
Combine chili powder, cumin, paprika, cayenne pepper and garlic powder in a small bowl. Set aside.
Meanwhile, mix 2 tablespoons of fajita seasoning into the ground beef.
Form the ground beef into 6 patties. Make a dimple in the center of each with your thumb and season with salt and pepper.
Toss the onion slices and peppers in olive oil and sprinkle with salt. Arrange the veggies in a grill basket and place on the cooler side of the grill over indirect heat, tossing occasionally until veggies are charred and softened, 7-10 minutes.
Arrange the patties on the hot side of the grill and close the lid. Grill 4-5 minutes per side until internal temperature reaches 160°F, flipping once halfway through for even cooking.
Place the buns on the cool side of the grill to toast.
When the burgers are almost done, place a slice of Manchego cheese on each burger. Close the lid to allow the cheese to melt. Remove from grill and rest the burgers.
Arrange the buns with lettuce and tomato on the bottom bun. Top with a burger, a dollop of guacamole and a healthy pile of fresh grilled onions and peppers.