Preheat the oven to 400 degrees F. Spray a 9x13 inch baking dish with nonstick spray.

Place the cauliflower florets in a microwave safe bowl. Add ⅓ cup of water and cover the bowl. Microwave until the florets are fork tender, about 6 to 8 minutes. Drain the water from the cauliflower bowl. Place the florets in the baking dish.

Make the sauce! Heat the butter in a saucepan over medium heat. Whisk in the flour to create a roux. Cook for 2 to 3 minutes, until the mixture smells nutty and fragrant. Stream in the milk while whisking the entire time. Cook for 5 minutes, whisking often, so the mixture can thicken.

Reduce the heat to low. Add in 1 cup of the grated gruyere and the parmesan. Stir until the cheese melts. Add in a pinch of salt and pepper and the nutmeg. Taste the sauce and add more salt and pepper as needed.

Pour the sauce over the cauliflower in the baking dish.

To make the mustardy breadcrumbs, tear the bread into pieces. Heat the butter in a nonstick skillet over medium heat. Whisk in the dijon. Add the bread pieces and toss, cooking for a few minutes until the bread is toasted and golden. Sprinkle the bread pieces on top of the cauliflower dish.

Bake for 30 to 35 minutes, until golden and bubbly and delicious. Sprinkle with fresh thyme and serve!