1. To make the harissa mayo, whisk together the mayonnaise, harissa, 1 teaspoon of the cumin, and vinegar or lemon juice in a medium bowl. Season with ½ teaspoon of the salt and ⅛ teaspoon of the pepper (or to taste). Set aside.*
2. To form the lamb burgers, combine the ground lamb, onion, garlic, remaining 1 teaspoon of the cumin, and coriander in a large bowl and mix gently with your hands. Form the mixture into 6 equal balls, each weighing approximately 5-6 ounces.
3. Flatten the balls into patties slightly larger than the bun you plan to serve with the burgers. (If not using a bun, flatten the balls into patties that are 1 to 1 and ½ inches thick.) Create a well or divot in the center of the burger with your fingers. This will ensure that the burgers remain flat and do not shrink or turn into round balls when cooked.**
4. Heat a cast iron skillet over medium-high heat until very hot, 2 to 3 minutes. Season the lamb burgers on both sides with remaining half teaspoon of the salt and ⅛ teaspoon of the black pepper.
5. Place 2, or at most, 3 patties in the skillet, being sure not to crowd them, and cook until the bottoms of the burgers are browned and release easily from the pan, 2 to 3 minutes.
6. Flip the patties and cook on the other side until browned, 2 additional minutes. Flip the burgers again and continue to cook, flipping every few minutes, until the internal temperature reaches 160°F. on an instant read thermometer (like this one), 8 to 10 minutes total. Repeat with the remaining patties.
7. To serve, place the patties on the bottom half of the burger buns. Top with your preferred burger toppings, such as lettuce, tomato, or onion. Spread the harissa mayo on the top half of the burger buns. Serve immediately.

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