If you prefer, this dish can also be made with two pounds of chicken or pork. Prepare and serve it the same way you would the shrimp. Feel free to add or omit any fresh vegetables or herbs based on availability.
Pulse garlic and salt in a food processor until coarsely chopped, 2 or 3 times. Add shrimp; pulse until shrimp pieces are the size of small peas. (Do not puree)
Heat the oil in a large skillet over medium-high heat. Add shrimp mixture and granulated sugar. Cook, stirring often, just until shrimp have cooked through, 3 to 5 minutes. Transfer mixture to a medium bowl. Let cool completely. Cover with plastic wrap, and refrigerate until ready to serve, up to 8 hours.
Heat fish sauce and the brown sugar in a small saucepan over medium-low heat, stirring constantly, until sugar has dissolved. Remove from heat. Stir in cayenne and lime juice. Set aside. (Dressing can be refrigerated up to 1 hour.)
Just before serving, add 1 sliced chile, the peanuts, and shallots to the shrimp mixture; toss to combine. Stir in dressing to taste; reserve any dressing that remains for dipping sauce.
Add remaining sliced and whole chiles and the ginger to the remaining dressing to make dipping sauce. Add equal parts fish sauce and lime juice to yield about 1/2 cup dipping sauce. Arrange lettuce, bean sprouts, scallions, cucumber, string beans, watermelon, herbs, and lime wedges on a platter. Serve with shrimp salad and the dipping sauce.