Melt the butter in a heavy-based pan until foaming, and stir in the flour to make a thick paste. Cook for a few minutes until it smells nutty. Gradually add the milk, a splash at a time, stirring continuously until you have a smooth, glossy saucy. Season.
Stir in the bay, cheddar, parmesan, half the taleggio and the whole tarragon leaves. Heat gently, stirring continuously for 6-8 mins until all the cheese has melted and the sauce has thickened. Remove from the heat and leave to infuse until ready to bake. To make ahead, leave to cool completely, cover the surface with a piece of baking parchment, and chill for up to 24 hrs. Reheat before using.
Cook the cauliflower florets in a steamer for 8-10 mins until just tender when pierced with the tip of a knife. If you don’t have a steamer, steam in 3cm water in a tightly covered pan. Tip into a baking dish and pour over the cheese sauce. Heat the oven to 220C/200C fan/gas 7. Stir in the chopped tarragon, then dot with the remaining taleggio. Bake for 20-25 mins until golden.