1. Preheat the oven to 350°F (175°C).

2. In a large skillet, cook the bacon over medium heat until crispy. Remove from the skillet and drain on paper towels. Crumble the bacon into small pieces.

3. In the same skillet, melt 2 tablespoons of butter over medium heat. Add the diced capsicum and cook until tender.

4. Meanwhile, peel and thinly slice the potatoes.

5. In a greased baking dish, layer half of the sliced potatoes. Sprinkle with salt and pepper.

6. Spread half of the cooked capsicum over the potatoes, followed by half of the crumbled bacon.

7. Repeat the layers with the remaining potatoes, capsicum, and bacon.

8. In a saucepan, melt 4 tablespoons of butter over medium heat. Whisk in the all-purpose flour until smooth.

9. Gradually whisk in the milk and cook until thickened, stirring constantly.

10. Remove from heat and season with salt and pepper to taste.

11. Pour the sauce evenly over the casserole.

12. Cover the baking dish with foil and bake for 45 minutes.

13. Remove the foil and continue baking for an additional 15 minutes or until the top is golden brown and bubbly.

14. Let it cool for a few minutes before serving.