How to Make It
Preheat oven to 375°F. Stir together sesame seeds, garlic, onion, poppy seeds, pepper, and 2 teaspoons of the salt in a small bowl.
Process oats in a food processor until finely ground. Add flour, sugar, baking powder, baking soda, garlic powder, onion powder, and remaining 1 teaspoon salt; pulse until combined. Transfer to a large bowl.
Whisk butter, buttermilk, and whole eggs in a medium bowl. Add to oat mixture; stir just until combined. (Mixture will be sticky.) Scrape onto a lightly floured work surface. Using floured hands, gently knead just until dough comes together, 3 or 4 times. Pat into a 9-inch square and cut into 9 3-inch squares. Transfer to a parchment-lined baking sheet.
Whisk egg yolk and 1 teaspoon water in a small bowl. Brush dough with egg wash and sprinkle evenly with seed mixture. Bake until well browned and center is firm to the touch, 20 to 22 minutes.