1. Preheat your oven to 425°F (220°C).

2. Slice the beef into thin strips and set aside.

3. Heat the butter in a skillet over medium-high heat and sauté the beef in batches until browned on all sides, seasoning with salt and pepper to taste.

4. Finely chop the leek and garlic.

5. Wilt the spinach in a separate skillet with a drizzle of olive oil over low heat until softened. Combine the cooked leek, garlic, and spinach in a bowl.

6. Thinly slice the potatoes and layer them in a greased baking dish, creating a uniform base.

7. Add the sautéed beef, leek mixture, and crumbled feta cheese on top of the potatoes.

8. To make the cheese sauce, whisk together 1 cup (250ml) of milk, 1/2 cup (120ml) of heavy cream, 2 tablespoons (30g) of all-purpose flour, and 1 teaspoon of Dijon mustard in a saucepan.

9. Bring the mixture to a boil over medium heat, then reduce the heat to low and simmer until the sauce thickens, stirring constantly.

10. Stir in 1 cup (115g) of grated cheese, 1 tablespoon of chopped basil, salt, and pepper until the cheese is fully melted and the sauce is smooth.

11. Pour the cheese sauce over the beef and potatoes, making sure it seeps between the layers.

12. Bake in the preheated oven for 30 minutes.

13. Remove the dish from the oven and top with cherry tomatoes, then return it to the oven for an additional 15-20 minutes, or until the top is golden brown and the sauce is bubbly.