Combine the sugar, water, and vinegar in a small saucepan. Bring the liquid to a simmer and allow the sugar to dissolve.
Pour the pickling liquid over the shallots, making sure all of the shallots are submerged.
Let sit for about 30 minutes, or until the burgers are ready. When ready to use, drain off the liquid. Store any remaining shallots in the liquid in the fridge for another use.
Combine the chopped parsley, dill, basil, oregano, and mint in a bowl. Stir in the olive oil, vinegar, garlic, lemon zest, and lemon juice.
Season with salt to taste. Set aside.
Heat the grill with one section over high heat and another over medium-low heat.
Place the ground lamb into a large mixing bowl. Add a heaping palmful of salt. Use your hands to break up the meat and mix well.
Form the meat into 12 flat patties (approximately 5.3 oz each). Place them onto a sheet pan and coat each one with a drizzle of oil.
Place the patties onto the hot part of the grill. Sear on both sides until slightly charred, about 1-2 minutes per side (depending on how hot your grill is). Transfer the patties to the medium-low heat. Cover the grill and cook for another few minutes until finished through.
Open the buns and place them onto the grill, cut-side down, for a minute or two until toasted.
Meanwhile, place a slice of gouda cheese on top of each lamb patty. Cover the grill again and let the cheese melt for just about 30 seconds.
To assemble each burger: spread mayo onto both sides of the toasted bun, place a small mound of watercress onto the bottom bun, place the burger with gouda on top, pile the shallots onto the burger, add a few radish slices, spoon the herb sauce over top, and close the top bun.