Place the cauliflower florets in a large microwave-safe bowl. Add ¼ cup of water. Cover and microwave on high until tender-crisp, about 7 minutes. Drain the cauliflower, then return it to the bowl and cover to keep warm.
In a medium saucepan, whisk together the heavy cream, Dijon mustard, garlic powder, and cayenne. Heat over low heat, stirring occasionally.
When the cream mixture starts bubbling on the edges, add the grated cheese gradually, mixing it in slowly and patiently with a rubber spatula.
Keep heating the cheese sauce over very low heat, stirring gently, until the cheese is completely melted and the sauce is smooth and slightly thickened.
Pour the cheese sauce over the cauliflower florets and gently toss to combine. Season with freshly ground black pepper and serve.
Video
Notes
If you feel that the cheese sauce is too thin, you can add ¼-1/2 cup more cheese. Alternatively, mix a teaspoon of cornstarch (gluten-free if needed) with a teaspoon of cold water, add it to the sauce, increase the heat to medium so that the sauce starts to gently bubble, and mix until thickened.
I don't add salt to this dish because Dijon mustard and cheddar are very salty. But you can add salt to taste if you wish.
You can keep the leftovers in an airtight container in the fridge for up to 4 days. Reheat them, covered, in the microwave. You can also freeze this dish for up to three months. I recommend freezing it in microwave-safe containers, in individual portions, and then microwaving those portions.