Preheat oven to 350 degrees.
Spread ½ cup rolled oats on a rimmed baking sheet and bake until light golden and fragrant, 8 to 10 minutes.
Cool then transfer to a food processor; process until finely ground.
Add flour, sugar, coarse salt, and pepper; pulse until combined.
Add butter, and pulse until mixture resembles coarse meal, about 10 seconds.
Add 3 tablespoons cold water and pulse until dough just comes together, adding up to an additional tablespoon as needed.
Turn dough out onto plastic wrap, shape into a rectangle and wrap in plastic.
Chill for 45 minutes.
Transfer chilled dough to a sheet of parchment paper.
Sprinkle with remaining ¼ cup oats and roll into a 10-by-12-inch rectangle.
Using a fluted pastry wheel, cut into thirty-two 2 ½-by-1 ½-inch rectangles.
Sprinkle with flaky sea salt.
If dough becomes soft, chill for 15 minutes.
Arrange bars an-inch apart on two parchment-lined baking sheets.
Bake until golden, 28 to 30 minutes.
Transfer to a wire rack to cool.
