Toss 1 diced English cucumber and 1 diced red bell pepper with 2 teaspoons kosher salt in a colander. Let drain 30 minutes, then pat dry. Meanwhile, grill 4 ears of corn over high heat, turning, until charred, 8 minutes; cut off the kernels. Bring 2 cups apple cider vinegar, 1/2 cup sugar, 1 tablespoon mustard seeds, 1/2 teaspoon ground turmeric and 1 tablespoon salt to a boil in a large skillet. Simmer until reduced to 1 cup, 8 to 10 minutes. Add the cucumber and bell pepper; cook 3 more minutes. Stir in the corn and 3 sliced scallions; refrigerate until ready to serve.