Preheat the oven to 375F (190C).

Cook the cauliflower florets until fork-tender, either in microwave steam bags following the package directions or in a steamer basket over simmering water for 6 to 8 minutes. Drain well and let the steam escape for a few minutes so the florets dry off.

Meanwhile, add the baby spinach, cream cheese and sour cream to a large saucepan over medium-low heat. Stir for 3 to 4 minutes, until the spinach has wilted and the cream cheese has melted into a smooth sauce.

Roughly mash the cooked cauliflower with a potato masher or fork - you don't want it super smooth, so leave some texture. Stir it into the creamy spinach mixture and season generously with salt and pepper.

Transfer the mixture to a baking dish and smooth the top. Sprinkle the shredded cheese evenly over the top.

Bake for 15 minutes, or until the cheese has melted and turned golden brown. Let the casserole cool for 5 minutes before serving.