1. Preheat the oven to 375°F (190°C).

2. Cook the brown rice according to package instructions.

3. In a large saucepan, melt the butter over medium heat. Stir in the flour to make a roux.

4. Gradually whisk in the milk until smooth and thickened.

5. Add the tuna to the saucepan and stir until well combined.

6. Season with salt to taste.

7. In a baking dish, layer the cooked brown rice and tuna mixture.

8. Bake in the preheated oven for 25-30 minutes, or until bubbly and golden brown on top.