Wash the cauliflower, towel dry, trim the outer leaves, and cut it into florets or chunks.
Place the cauliflower florets in a microwave-safe bowl. Add 4 tablespoons of water to the bottom of the bowl, cover it with a microwave-safe plate, and microwave on high for about 10 minutes, until very tender.
Meanwhile, place the butter, garlic, parmesan, salt, and pepper in your food processor's bowl.
When the cauliflower is cooked, carefully drain it (the bowl and plate will be hot, and steam will escape) and transfer it to the food processor.
Process until very smooth, 1-2 minutes, stopping once or twice to scrape the sides of the bowl with a spatula. Serve immediately.
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Notes
It's perfectly acceptable to use frozen cauliflower florets in this recipe. The instructions are the same as using fresh cauliflower.
Don't undercook the cauliflower. For this recipe, you want it to be very soft, not tender-crisp.
Drain the cauliflower well after steaming it. The less water it contains, the thicker the mash will be.
Make sure you add dry-grated parmesan. Its texture nicely thickens the mash.
You can keep the leftovers in the fridge, in an airtight container, for 3-4 days. Gently reheat them in the microwave, covered, at 50% power.