Preheat your oven to 375°F (190°C). Lightly grease a 9x13 inch baking dish.
Steam cauliflower florets until just tender, about 5-7 minutes. Don't overcook - they'll continue cooking in the oven.
In a large skillet, sauté mushrooms over medium-high heat until they release their moisture and turn golden brown, about 8-10 minutes.
In a saucepan, melt butter over medium heat. Add garlic and cook until fragrant, about 30 seconds.
Whisk in flour and cook for 1 minute to remove the raw flour taste.
Gradually pour in milk and cream, whisking constantly to prevent lumps. Simmer until sauce thickens.
Stir in 1 cup of gruyere cheese until melted. Season with salt and pepper.
Layer the cauliflower and mushrooms in the prepared baking dish.
Pour the cheese sauce evenly over the vegetables.
Mix remaining cheese with panko and thyme, then sprinkle over the top.
Bake for 25-30 minutes until golden brown and bubbling.