Cheese and Onion Scrumps

From Syllabub's blog.

• Ready In: 50 mins
• Yields: 18 crisps

Ingredients

• 7  tablespoons unsalted butter, kept very cold and cut into chunks
• 3⁄4 cup all-purpose flour, plus
• 1  tablespoon all-purpose flour
• 1  pinch salt, a good large pinch
• 1⁄2 teaspoon mustard powder
• 1⁄2 cup  coarsely grated aged cheddar cheese
• 1⁄2 cup  finely grated parmesan cheese
• 1 -2  tablespoon nigella seeds (black onion seeds)
• 1  egg yolk, beaten

Directions

1. Combine all ingredients in a food processor. Pulse until ingredients come together in a loose ball. Turn out onto plastic wrap. Work the mass together with the tips of your fingers until blended. Form into a log and wrap tightly. Chill in refrigerator at least 30 minutes, up to overnight.
2. Preheat oven to 350°F Line a baking sheet with parchment or Silpat.
3. Dip the tip of a knife into a mug of very hot water, then cut thin slices off the log. Place on baking sheet about 1" apart. Bake 10 minutes or until golden and crisp. Remove to a rack to cool.