In a pan of hot drippings (if serving with a protein), add 1 tablespoon of butter and a little water
Add leeks. Cook, stirring often, for 3 to 4 minutes. Add water as needed.
Add garlic and cook for about 1 minute
Add flour and cook for 1 to 2 minutes
Add milk, and rosemary and cook until it comes to a boil and thickens (3 to 5 minutes).
Remove the rosemary
Add the spinach and wilt
Cook until spinach reaches desired softness
Season with salt and pepper