Make the burgers. Add lamb, mushrooms, salt, pepper and soy sauce to a bowl. Mix until just combined. Divide into 8 equal patties, about 6 ounces each. Place patties onto sheet pan lined with parchment paper, cover with plastic wrap and let rest at room temperature for 30 minutes.
Make the aioli. Combine harissa, mayonnaise and salt in a bowl. Whisk until incorporated. (leftover aioli can be stored in the refrigerator for 7 days.)
Make the marmalade. Heat small sauce pan over medium-high heat. Add red wine vinegar and honey and reduce until syrupy. Set aside. In a separate skillet over medium heat, add sliced onion and salt. Cook for 2 minutes or until the onions are translucent, stirring occasionally. Add sugar and chili flakes and continue to cook until the mixture is golden brown and caramelized, about 10 minutes. Remove from heat. Stir in vinegar and honey mixture until incorporated. Stir in chopped mint. Set aside.
Heat grill to 350 F. Sprinkle seasoning salt over top of each patty and cook for 3 minutes each side. (Be sure to not touch the burgers while they are cooking as this will keep the juices inside.) Top each patty with goat cheese, close the lid of the grill and cook until cheese is melted, about 1 minute. Remove patties from grill and let rest for 2 minutes before assembling. While meat is resting, heat or toast the buns to desired doneness. Spread harissa aioli on one half of the bun and top with tomato, burger patty, sweet onion marmalade and butter lettuce. Spread harissa aioli on other half of bun and place on top of lettuce.