Marinated Cucumber and Tomato Salad

Name: Marinated Cucumber and Tomato Salad
Prep time: 15 min
Total time: 15 min
Cuisine: American
Ingredients: 3 medium cucumbers, peeled and sliced 1/4 inch thick (for about 4 cups sliced); 1 tablespoon kosher salt or sea salt; 1/2 cup white wine vinegar; 1/2 cup rice vinegar; 1/3 cup granulated sugar, or to taste; 1 teaspoon mustard seed; 2 cups thinly sliced Vidalia or other sweet onion, halved and cut into half moons; 3 cups chopped tomatoes, juices retained; 1/2 teaspoon freshly ground black pepper, or to taste; 1/8 teaspoon dried red pepper flakes, or to taste, optional