In a food processor, combine all hummus ingredients, 1 tablespoon olive oil, and ½ teaspoon salt and blend until smooth. Set aside until ready to use (if preparing burgers later, store hummus in an airtight container in the refrigerator.)
Preheat the oven to 450 degrees F. In a medium bowl, combine lamb, ½ teaspoon salt, ½ teaspoon pepper, and mint, and form into two burgers. Place two slices of onion on a baking sheet and place one burger on top of each. Top each burgers with a dollop of harissa and two more slices of onion. Roast for 17 minutes or until meat reaches an internal temperature of 145 degrees F.
While burgers cook, assemble salad ingredients and divide between plates. Garnish burgers with additional harissa and feta and serve with salad.