Preheat oven to 425°F. Drizzle garlic with oil and season with a little salt. Wrap tightly in foil and roast garlic to soft, 45 minutes. Let cool to handle.
Combine garlic, half-n-half, salt, white pepper and a little nutmeg in a blender and purée. Adjust seasoning to taste.
In medium skillet over medium heat, melt butter and whisk in flour. Cook 1 minute, add half-n-half mixture and bring to a bubble. Separate spinach with your fingertips as you add to the sauce. Simmer at a low bubble to thicken, 20-25 minutes.