Preheat the oven to 375°F (190°C). Butter a shallow baking dish. Bring water to a boil in a steamer pan. Arrange the cauliflower florets on the steamer rack, place over the boiling water, cover, and steam until tender when pierced with a fork, about 15 minutes. Do not overcook. Remove the florets from the steamer and arrange snugly in a single layer in the prepared baking dish. Set aside.
Meanwhile, make the sauce. In a saucepan over medium-high heat, melt the 4 tablespoons butter. When it foams, remove the pan from the heat and whisk in the flour to make a thick paste. Return to the heat and slowly add the milk, whisking constantly. Add the cayenne and ½ teaspoon salt, reduce the heat to medium, and simmer, stirring occasionally, until the sauce is thick enough to coat the back of a spoon, about 15 minutes. Stir in half of the Gruyère and the Mimolette. Taste and adjust the seasoning with salt, if needed. Pour the sauce over the cauliflower in the baking dish. Sprinkle with the remaining Gruyère.
Bake until the top is golden and the sauce is bubbling around the edges, about 20 minutes. Serve hot.