Do ahead:

• Pulse flour, oats, pistachios, salt, and baking soda in a food processor to combine (small pieces of oats and pistachio should still be visible).

• Using an electric mixer on medium-high speed, beat butter, granulated sugar, and powdered sugar in a medium bowl to create air pockets in butter, about 5 minutes; mixture will look light and fluffy. Reduce speed to low and add dry ingredients, mixing just until blended. Divide dough between 2 sheets of plastic wrap or parchment paper and shape into about 1½”-diameter logs. Chill until firm, at least 1 hour.(The colder your dough, the easier it will be to cut into even slices.)

• Place racks in lower and upper thirds of oven; preheat to 350°. Using a serrated knife, slice logs into ¼”-thick rounds and place on parchment-lined baking sheets, spacing about ½” apart. Bake, rotating baking sheets once, until cookies are golden brown around the edges, 10–12 minutes. Transfer to wire racks; let cool.

  For a delicate—not brittle—crunch, don’t overbake. It’s okay if the center of the cookie is still pale.

• DO AHEAD: Keep dough chilled up to 5 days, or freeze up to 1 month.