For the turkey burgers: Coat a large saute pan with olive oil and toss in the onion. Season with salt and bring the pan to medium heat. Cook the onion until soft and very aromatic, 7 to 8 minutes. Add the garlic and cook for another 1 to 2 minutes. Turn off the heat and let cool.
In a large mixing bowl, combine the turkey, water chestnuts, soy sauce, cilantro, ginger, sambal if using and the cooked onions and garlic. (NOTE: Save the onion pan to cook the burgers in later!) Add 1/4 to 1/2 cup water to the mix--this will keep the burgers really moist! Use your hands to squish the mixture until everything is really well combined.
Make, cook and eat a little tester patty to be sure the turkey is really delicious. If the seasoning isn't just right, add a little more soy or a sprinkey-dink of salt. When you're confident the burger mix is perfectly seasoned, form it into 4 equal patties.
With a paper towel, wipe out the saute pan you used to cook the onion, coat the pan with fresh olive oil and bring it to medium-high heat. Add the burgers, being sure not to crowd the pan--if you need to work in batches, knock yourself out. Cook the burgers for 5 to 6 minutes on each side. The burgers should be browned and cooked through when done. (If working in batches, keep the first batch warm in the oven at 200 degrees F while cooking the second batch.)
For the toppings: Mix together the mayo and sambal oelek in a small bowl. Toast the burger buns and top with the burgers. Garnish as desired with the cheese, tomato, red onion, lettuce and spicy mayo.