Rub the peppers with just enough oil to coat. Working with 1 poblano pepper at a time, place the peppers directly over a medium-high flame and char. Allow the peppers to get black and blistery. Place in a plastic bag and allow to steam for 15 minutes.
Coat a large skillet with oil set over medium heat. Add the diced onions, 1/4 teaspoon salt and red pepper flakes, stirring occasionally, cooking just until the onions begin to soften, about 3 minutes. Add the garlic and cook for 2 minutes. Remove from heat and cool.
Using a paper towel, rub off the black charred skin of the peppers. (Optional: Cut a small slit in the peppers to remove the seeds.) Cut the peppers into thin strips. Set aside.
In a large mixing bowl, combine the onion mixture with the lamb, 1 teaspoon salt, pepper, cumin, mint, oregano and lemon zest (It is easiest to use your hands).
Divide the lamb mixture into 6 equal pieces. Form into patties. Use your thumb to make a dimple in the center. This will make sure your burger cooks in the middle. Place on a cookie sheet and chill.
Coat a skillet with oil. Cook the onion rings over medium heat, stirring occasionally. Allow them to caramelize, stirring occasionally. Cook for 5 to 8 minutes. Add the poblano peppers the last 3 minutes. Remove from the heat and set aside.
Using the same skillet, coat with oil and cook your patties over a medium flame for 4 to 5 minutes each side for medium-rare. Allow to rest for 3 minutes before serving.
Build your burger: Mix the mustard and mayonnaise until well blended. Spread some on both sides of the buns. Place some arugula on the bottom bun and top with the lamb patty. Add a slice of cheese. Pile on the onion and poblano pepper slices and top off with the hamburger bun.