How to Make It
Stir together sourdough starter discard, cornmeal, flour, seeds, kosher salt, and 2 tablespoons of the oil in a medium bowl. Knead gently just to bring dough together. Divide dough in half, and shape into 2 (about 4-inch square) bricks. Wrap each brick tightly in plastic wrap; chill until firm, at least 30 minutes or up to 3 hours.
reheat oven to 350°F. Cut a sheet of parchment paper to form an 18-by-13-inch rectangle; lightly dust with flour. Unwrap 1 dough brick, and place on prepared parchment paper. Lightly dust top of dough brick, and roll to a 14-by-12-inch rectangle, about 1/16 inch thick. Brush top of dough rectangle with 1 1/2 tablespoons of the oil. Sprinkle with 1/4 teaspoon of the flaky sea salt (or more to taste), and optional toppings.
Using a pizza cutter or a chef’s knife, cut dough rectangle into roughly 1 1/2-inch squares. You’ll have about 60 squares. Carefully slide parchment paper with dough squares onto a baking sheet. Bake until crackers are golden around edges, 18 to 24 minutes, rotating baking sheet 180 degrees halfway through bake time. Transfer baking sheet with crackers to a wire rack; let cool completely, about 30 minutes. Meanwhile, repeat process with remaining dough brick, oil, flaky sea salt, and flavoring sprinkles. Crackers can be stored in an airtight container at room temperature up to 1 week.