Preheat oven to 400 degrees F.
Toss cauliflower florets in olive oil and place on baking sheet.
Season with salt and pepper.
Roast in oven for 20 minutes.
In medium skillet, melt butter over medium-high heat.
Add shallots and cook for 5-7 minutes, until softened.
Add garlic and cook another minute.
Add flour, and stir well with shallots and garlic.
Cook 1 minute.
Slowly add cream followed by milk, stirring constantly.
Season with sage, nutmeg and salt and stir well to combine.
Bring to a simmer, reduce heat to low, and let simmer for 20 minutes, until thickened.
Remove from heat and stir in cheese, stirring until melted.
Place roasted cauliflower in 9" baking dish. I used a 9" cast-iron skillet, but any similar size will work.
Pour cheese sauce over cauliflower evenly.
Combine breadcrumbs, Parmesan cheese, and butter, then sprinkle over baking dish.
Bake in preheated oven for 20-25 minutes, until breadcrumbs are golden.
Optional: fry the sage leaves in oil. Do this by heating ~2 tablespoons of vegetable oil over medium-high heat. Once hot, add the sage leaves and fry for 15-20 seconds, until they turn dark green. Remove from skillet immediately.
Top gratin with fried sage.