4 ounces cheddar, coarsely grated
2 ounces Parmesan, finely grated
3/4 cup all-purpose flour
1/4 teaspoon dry mustard
1/4 teaspoon kosher salt
1/8 teaspoon cayenne pepper
4 tablespoons (1/2 stick) unsalted butter, softened and cut into small pieces
2 tablespoons water, plus more if needed
In the bowl of a food processor, place all ingredients except the butter and water. Pulse 5 times. Add the butter and pulse again until the butter pieces are the size of BBs. Add the water, 1 tablespoon at a time, and pulse just until the dough holds together. If the dough is still crumbly, add more water, 1 teaspoon at a time, until it reaches the right consistency.
Turn the dough out onto a large piece of waxed paper. Roll the dough into a log, 9 to 10 inches long, and square off the ends. Refrigerate, well wrapped, for at least 2 hours and up to 2 days. Or freeze it for up to 1 month. (You may want to cut the log in half or in thirds to freeze if you think you will want to defrost a smaller amount at a time.)
Preheat the oven to 375 degrees F.
To make the crackers, cut the log into 1/4-inch-thick slices. Arrange the slices on a baking sheet 1-inch apart. Bake for 8 to 10 minutes, or until the crackers are a light golden color. Turn the crackers and bake for 3 to 5 more minutes, or until they are golden around the edges. Cool on a rack. Serve at room temperature.