Trade trendy for traditional with a crisp, savory snack that's easily customizable.
In a food processor, pulse together butter, cheese, egg, mustard, salt, and red-pepper flakes until smooth. Add flour and pulse until combined. Transfer dough to a work surface and form into a 2-inch-wide log. Wrap in plastic and refrigerate until firm, 4 hours (or up to 1 day).
Preheat oven to 350 degrees, with racks in upper and lower thirds. Slice dough into 1/4-inch rounds and arrange, 1 inch apart, on 2 parchment-lined baking sheets. Bake until edges are golden, 15 to 20 minutes, rotating sheets halfway through. Let cool completely on racks before serving.
Store crackers in an airtight container, up to 1 day.
In place of red-pepper flakes, try stirring in 3 tablespoons tomato paste or 1/3 cup crumbled cooked bacon, finely chopped herbs, or finely diced fresh green chiles.