Preheat the oven to 370 F. Fry the bacon strips in a pan until crispy for about 7 minutes. Set aside. If using frozen green beans just leave on counter for 20 minutes or put into a bowl to heat in microwave for 30 seconds to 1 minute to thaw a bit until frost on outside is gone.
Mix green beans, salt, sugar, paprika and olive oil in a bowl. Transfer to a baking dish, such as 8x11 inches. Top with crispy bacon. Bake in the oven for 25 minutes.
While the green beans and bacon are baking, melt the butter in a saucepan and whisk in all-purpose flour so it is smooth. Stir and warm for 2 minutes on low heat.
Add milk little by little and continue stirring the sauce with a whisk. You need to stir all the lumps and add salt. If the sauce is too thick, add more milk. If desired, you can add grated Parmesan to the sauce to make more rich.
After 5 minutes, remove the sauce from heat. Remove the crack green beans and bacon from the oven. Serve with sauce poured over the top.