Preheat the oven to 350°F (176°C).

Prepare ingredients. Coat a 10-inch pie dish or cake pan with non-stick spray and set aside. Cube the butter into half-inch chunks and keep chilled in the freezer. Combine granola and walnuts in the food processor and process on high speed into a fine crumb, but not a paste.

Combine the dry ingredients. Add the granola, oats, walnuts, rice flour, granulated sugar, and kosher salt to the bowl of a food processor. Pulse 10 to 12 times until the mixture is pebbly and fine.

Add the butter. Pulse the mixture another 8 to 10 times until the mixture feels like damp breadcrumbs. The butter should almost “disappear” into the dry ingredients, but not taken so far that the dough is clumping and gathering around the blade.

Press into mold. Transfer the dough to the prepared baking tray and scatter evenly. Use the backs of your knuckles to lightly press the crumb into an even layer. It should be about a half-inch tall. Do not apply too much pressure as this will make the shortbread dense.

Bake and then score. Bake until the edges are lightly brown and the center feels soft but baked through, about 35 to 40 minutes. Remove from the oven and sprinkle generously with flaky sea salt. While still hot, score into desired shapes using a small knife or cookie cutter. Let cool completely, slice or break cookies into narrow slices, and dust with confectioner’s sugar.