Preheat your oven to 425 degrees F. Grease a shallow 1.5-quart baking dish with butter or olive oil.
Place the cauliflower florets in a large microwave-safe bowl. Add a tablespoon of water. Cover with a microwave-safe plate and microwave on high for 7-8 minutes, until just tender.
Meanwhile, in a large bowl, mix together the sour cream, mustard, garlic, gruyere, salt and pepper.
Drain the cooked cauliflower well and pat dry. Add it to the sour cream sauce, tossing to combine.
Transfer the mixture to the prepared baking dish. Sprinkle the Parmesan on top and bake until heated through, about 10 minutes.
If your baking dish is broiler-safe you can switch the oven to broil after the gratin is done baking and broil the dish for 1-2 minutes to brown the top. Keep a close eye on it so it doesn't burn. You can keep the leftovers in the fridge, in an airtight container, for up to 4 days. Reheat them gently, covered, in the microwave on 50% power.