In a large bowl, stir mint, lemon zest, garlic, oregano, paprika, cinnamon, cumin, cayenne pepper (if using) and 1 teaspoon Kosher salt.

Add lamb and mix with your hands until just combined (do not overmix). Shape mixture into four equal patties (about 4 inches across and 3/4-inch thick). Transfer burgers to a vacuum sealed bag or ziplock freezer bag (remove as much air as possible).

Heat sous vide machine to 135°F for medium-rare. Once the water is heated, add the burgers and cook for 45 minutes. You can leave the burgers in the water bath for another 1-1/4 hours if that works better for your schedule. (That means they’re fine in the bath for 2 hours.)

While the lamb burgers are cooking, prepare the balsamic onions. Heat a medium skillet over medium heat and add olive oil.

Add onions and sprinkle with 1/4 teaspoon Kosher salt. Stir occasionally until the onions begin to soften and brown a little, about 10 minutes. Don’t rush – it’s okay if it takes a bit of time.

Stir in the brown sugar and cook for another 3 minutes.

Add balsamic vinegar and stir to deglaze the pan. Lower the heat to medium-low and simmer mixture for about 5 minutes until the onions are soft and jammy. Season with Kosher salt and freshly ground black pepper. It’s okay if they cool slightly before serving as they will thicken up a bit.

Remove the cooked burgers from the bag and pat dry with a paper towel.

Heat a gas grill on high and cook the patties for 1 to 2 minutes on each side.

Place each patty on a bed of arugula or small hamburger bun. Divide balsamic onions and crumbled feta cheese between the four burgers. Serve warm.