- Halve the chiles lengthwise and trim out their cores and white pith (the seeds add extra heat, so remove them, too, if you prefer a milder flavor). Slice the onions into 1/2 inch rings. Lightly coat all the vegetables with the olive oil and season with salt and pepper.
- Before you make the burgers, ensure that the meat is very cold in the refrigerator. If you’re in a hurry, you can spread it in a single layer on a rimmed baking sheet and freeze for 10 or 15 minutes, just until it’s well chilled all the way through.
- Taking care not to over handle the meat, shape it into 6-ounce patties that are a little wider than the buns. Use your fingers to press a slight indentation in the center. Set the formed patties aside to let come to room temperature, then brush with olive oil and season with salt and pepper.
- Prepare your grill so there’s a medium-high flame on one side and low heat on the other side. Clean the grate and let it get warm for at least 5 minutes.
- Grill the chiles and onions directly over the flame. Flip the chiles once they’re slightly blistered and charred, approximately 10-15 minutes, and repeat on the other side. The onions are ready to flip when they have distinct grill marks and are just beginning to soften but still have some crunch, 4 to 5 minutes; if they’re not held together with skewers, take care as you flip so they don’t slip through the grates. Remove onions from grill and set aside. When the peppers are cool enough to handle, rinse them under water to peel away their papery charred skin, pat them dry, and julienne. Set aside.
- Grill the burgers, indentation side up, over a medium-high flame. Cook undisturbed for at least 4 minutes; check the bottoms by carefully lifting up an edge with your spatula and flip when you see good grill marks. While the burgers are cooking, in a separate bowl, mix together the mayonnaise, Mexican crema and lime zest. Set aside.
- Top each patty with a big spoonful of the chiles, then blanket them with a slice of cheese and continue to cook for 3 to 5 minutes, depending on desired doneness: a medium-rare burger will have just a little ‘give’ if you press it with your finger, while a medium burger will be pretty firm. If the outsides are seared before the inside is cooked through, move the patties so they’re just outside the flame to slow their cooking. Once they’re done, move them to the coolest part of your grill to keep them warm.
- Lightly toast your buns on the grill, over indirect heat, for one to two minutes.
- To assemble the burgers, spread a layer of the crema mixture on both buns. Starting with the bottom bun, add the patty, grilled onions and lettuce and top with the other bun. Serve immediately.