Preheat oven to 350°. Line a baking sheet with parchment paper or silicone baking mat.
In a food processor or blender, combine tuna, egg, and flour and process until until a thick paste forms.
Spread paste evenly over about an 8x8 square on your prepared baking sheet (it doesn't need to be exact or pretty). Bake for 15 minutes, until mixture is set.
Store in an air-tight container in the fridge for up to 1 week or in the freezer for up to 3 months.