In a skillet, heat oil over medium heat. Once warmed, add the flour and whisk fast (to prevent flour from burning). Add thawed spinach (liquid and all) and whisk until incorporated. Then add the egg and continue whisking for about 3-5 minutes until the spinach is hot and egg cooked.
Add the garlic powder, sour cream and milk, whisk until incorporated and cook for about 5 more minutes, stirring occasionally. If the sauce is too thick, add more milk and let it simmer. Add a teaspoon of salt, then taste to see if you need more. Set aside.
Place a large pot of salted water on the stove and bring it to a boil, then set aside.
Meanwhile make the béchamel
In a small saucepan, set over medium heat, melt the butter. Immediately add the flour and whisk until you get a smooth paste. Slowly add the warm milk, stirring constantly.
Let the béchamel cook for about 3-5 minutes, keep on stirring until you get a smooth but thick sauce (like a thick creamy soup). Season with salt, nutmeg and few grinds of black pepper and set aside.