For Burgers
(cook while peppers are grilling)
In a medium bowl, mix in the ground beef and mushrooms together. Add the spices, breadcrumbs, and egg and continue to mix until well combined. Divide into 4 patties.
Do not cook burgers until the poblanos peppers are finished roasted. Grill on a skillet for 7 minutes on each side until medium rare. Keep burgers warm in oven.
For Chile Rellenos
Preheat oven to broil. Place large poblano peppers on a baking sheet lined with foil and lightly oil the peppers. Place baking sheet on second to highest level in oven. Broil for 10-12 minutes rotating the pepper every 3 minutes. The skin on the pepper will start bubbling, and gets toasted and blackens. No burning though!
As soon as the peppers are removed from oven, place peppers in a plastic ziplock bag to allow peppers to sweat, allowing the pepper skin to soften. After 10-15 minutes, remove peppers, and remove outer pepper skin with a paper towel. Using gloves and a knife, carefully remove the stem and empty some of the seeds. This keeps the pepper a bit spicy. If you prefer a less spicy pepper, then remove all seeds.
Split the pepper in half and fill with 1/3 cup of mozzarella cheese and close the pepper, repeat with second pepper. Heat a medium nonstick pan with 1 Tablespoon olive oil on medium heat. Grill peppers for 5 minutes on each side or until cheese is melted. Cut each pepper in half for a total of 4 chile rellenos.
For tomato sauce:(prepare while roasting peppers)
In a medium skillet, saute onions and tomatoes in olive oil over medium-high heat for 7 minutes.Add garlic and cook for another 5 minutes. Add the grilled chile relleno to the tomato sauce.
For crema sauce: (topping)
Mix crema or sour cream, lemon juice, garlic and salt until well combined.
Assembly
Toast the potato buns. Add one burger patty to base of bread. Top each burger with a chile relleno with tomato sauce. Add a dollop of crema on top and close with a hamburger top. Enjoy!