In a small bowl, toss daikon and carrot with 1/4 teaspoon salt. Let sit 5 minutes, then squeeze out excess moisture. Add rice vinegar, sugar, and remaining 1/4 teaspoon salt. Stir to dissolve, then set aside for at least 15 minutes. Drain before using.

In a large bowl, gently combine ground turkey, fish sauce, minced garlic, and black pepper. Do not overmix. Divide into 4 equal portions and shape into patties about 3/4 inch thick. Press a slight dimple in the center of each patty to prevent puffing.

Heat oil in a grill pan or outdoor grill over medium-high heat. Cook patties 4-5 minutes per side until deeply browned and internal temperature reaches 165°F. Let rest 3 minutes.

While patties rest, toast the cut sides of the baguette sections until golden and crisp, about 1-2 minutes.

Stir together mayonnaise and sriracha. Spread on both cut sides of toasted baguette. Layer bottom half with drained pickled vegetables, cucumber slices, turkey patty, cilantro, and jalapeno if using. Close with top half and serve immediately.