Pre-cook the cauliflower florets and peeled, diced potatoes in salted water for 10 minutes, then drain. Spread in a deep, greased casserole dish, pour the tomatoes on top. Preheat the oven to 180-200 degrees.
For the sauce, cut the leek into thin rings. Sauté together with the garlic in the butter for 1 minute, stir in the flour and sweat for another minute. Gradually add the milk and stir well. Season with salt, pepper and paprika and stir in 50 g cheese, bring to the boil. Spread the sauce over the vegetables, sprinkle with the rest of the cheese and bake until the cheese browns.