If you are looking for a gluten-free cereal, this is for you. I like the addition of almond flour to add another flavor profile to the flakes, but you could also try other gluten-free flours, like amaranth or sorghum, for a different flavor outcome.
Preheat the oven to 350 degrees F. Line a rimmed baking sheet with parchment paper and lightly spray with oil.
Put the rolled oats in a food processor and pulse to finely grind into oat flour.
Whisk together the oat flour, almond flour, brown rice flour, light brown sugar and salt in a medium bowl. Mix the vanilla with the water in a liquid measuring cup, then slowly pour the liquid in a stream into the flour mixture, stirring constantly, until it becomes a pourable batter (like a thin pancake batter); add more or less water as necessary. Pour the batter onto the prepared baking sheet, then use an offset spatula to spread the batter evenly all the way to the edges, smoothing the top so that it is quite thin (you can also use a rolling pin and roll the batter out between two sheets of parchment paper). It should be no more than 1/4 inch thick.
Bake until the top is dry and begins to crack, 15 to 18 minutes. Let cool until it is cool enough to handle, 10 to 15 minutes. Reduce the oven temperature to 250 degrees F.
Break the baked layer into 1/2- to 1-inch pieces. Spread the broken pieces evenly on the same baking sheet. Bake, stirring halfway through and redistributing on the baking sheet, until the flakes are dried and crisp, 45 to 50 minutes. Allow to cool completely. The cereal can be stored in an airtight container at room temperature for up to 2 weeks.
Instead of processing the oats, you can also use 1 cup of store-bought certified gluten-free oat flour.
If you do not want the cereal to be gluten-free, you can use all-purpose flour instead of the brown rice flour or gluten-free flour blend.