Preheat your oven to 375°F (190°C). Butter a 9x13 inch baking dish.
Steam the cauliflower florets until just tender, about 5-7 minutes. Don't overcook – they'll continue cooking in the oven.
In a large saucepan, melt 2 tablespoons of butter over medium heat. Add flour and whisk for 1-2 minutes to create a roux.
Gradually pour in the milk while whisking constantly to prevent lumps. Cook until the sauce thickens, about 5 minutes.
Remove from heat and stir in half of both cheeses, nutmeg, salt, and pepper.
Fold in the spinach until it wilts slightly, then gently stir in the cauliflower.
Transfer the mixture to your prepared baking dish.
In a small bowl, combine breadcrumbs with remaining cheese, melted butter, and garlic.
Sprinkle the breadcrumb mixture evenly over the top.
Bake for 25-30 minutes until golden brown and bubbling.