Bring a large pot of water to a boil over high heat. Add the cauliflower and cook until the florets are tender, about 8 minutes. Drain well.

Mean while,make the sauce:

In a large saucepan over low heat, melt the butter and add the onions. cook for about 5 minutes. Add the flour a little at a time, whisking to incorporate. Increase the heat to medium-low and gradually whisk in the milk.

Cook, stirring frequently, until the mixture is thick and creamy, about 5 minutes. Add the cream,mustard and cheeses.

Add the cumin , paprika, salt and black pepper. Cook, stirring frequently, until the cheeses are melted, about 2 minutes.

Preheat the oven to 450F. Butter a shallow baking dish. Arrange the cauliflower in the dish and pour the sauce over it. Bake until golden brown, about 7 minutes. Serve immediately.