Set the Greek yogurt out to come to room temperature. Place a rack in the center of your oven and preheat to 375°F. Generously coat a 9x13-inch baking dish with nonstick spray.

Place the cauliflower in a large microwave-safe bowl and add 1 inch of water. Cover with a damp paper towel and microwave on high power for 4 minutes. Stir, then recover and microwave for 4 minutes more, until the cauliflower is slightly softened but still definitely fairly al dente. Drain into a colander, then spread into a single layer on paper or kitchen towels and pat dry. Let air dry while you make the sauce.

In a Dutch oven or large, deep stockpot, heat the butter over medium heat.

As soon as the butter melts, sprinkle the flour over the top and cook, whisking constantly, until flour turns light golden brown and smells lightly nutty, about 1 minute. Gradually pour in the milk, whisking constantly to prevent lumps from forming. Bring to a simmer and cook, stirring continuously to make sure the sauce doesn’t stick to the bottom, until the sauce thickens, 3-5 minutes.

Reduce the heat to low. Whisk in the mustard, salt, black pepper, garlic powder, and cayenne pepper.

Next, stir in the cheddar by small handfuls, adding it gradually and stirring all the while to ensure it melts smoothly. Make sure each addition of cheese melts before adding the next.

Remove from the heat. Stir in the Greek yogurt.

Add the cauliflower, stirring gently to combine. The sauce will seem very thick, but will thin out as water is released from the cauliflower when it cooks. Transfer the mixture to the prepared baking dish and spread into an even layer.

Bake for 25 to 35 minutes, or until hot and bubbly and the cauliflower is just fork tender. If you’d like to crisp the top, turn the oven to broil for a few minutes until golden in spots. Let rest 5 minutes, then serve.