Wash and dry peppers, slice in half and remove membrane and seeds. Grill the peppers directly over high heat on a grill or place the peppers on a foil-lined baking sheet, skin side up, about 6-inches under the broiler in the oven and broil on high until well blistered and dark brown/black, 8-10 minutes. Turn peppers halfway through to blister both sides.
Once roasted, place peppers in a plastic bag to steam, about 15 minutes. This helps loosen the skin, making it easier to remove. Remove and discard skin before dicing peppers.
For the Salsa Mayo
Mix the mayonnaise and drained salsa together, taste and season with salt and pepper. Set aside until ready to serve.
Grilling the Onions
Heat grill to at least 425°F. Arrange sliced onions on a rimmed baking sheet, drizzle with olive oil, then generously season with salt and pepper.
Grill onions over high heat until softened and browned on both sides, about 8 minutes. Remove to a platter and cover with aluminum foil. Note: Onions can be grilled directly on the grill grates, but a vegetable grill pan makes it easier.
For the Burgers
Gently mix ground beef with diced roasted chilies, cheese, and dry spices until blended. Pat into 1/3 lb burgers.
Grill burgers over med-high heat (dimple side up) for about 4 minutes. You'll see the color of the burger changing on the outside edge and juices begin to collect in the dimple. Turn and cook on the other side another 4-5 minutes, or until the internal temperature is between 155-160°F. NOTE: If adding more cheese on top, do so the last minute of cooking time.
If there is room on the grill with the burgers, toast buttered buns (butter side down) until lightly browned with crisp edges. Otherwise, toast once burgers are done and resting.
Serve the burgers on toasted buns with the sauce and grilled onions on top.
NOTE: It is important to use gloves when handling peppers.The type of chile pepper you choose will affect how spicy the burger is, as well as the type of salsa used in the mayo. I like to use a mild salsa and spicier chilies so the mayo has a cooling effect.
Season with Green Chile Rub (recipe below) for an extra kick of green chile spice.
Use a mix of different chiles to include Poblano, New Mexico, and Jalapeno, or mix in chipotle chiles in adobo for added smoky heat.
Substitute canned or jarred green chiles for fresh.
Top with slices of crisp bacon.
Make it Paleo or low-carb and serve on lettuce and sliced tomato instead of a bun.