Mash potatoes in a bowl and 1 cup of the flour, reserving the rest to use if the gnocchi is not light and fluffy. Season with salt and pepper, to taste. Add beaten egg and cheese and mix everything well. Remove half the mixture to a separate bowl and stir in the chopped spinach. Knead the ingredients in each bowl lightly and on a floured board form into long cylinders. The gnocchi should have a light texture which is cohesive enough to remain whole while it is being boiled. If the mixture in either bowl is too wet, add the additional flour until the proper consistency is achieved. Conversely add milk if the dough is too dry, using only enough as needed to bring the dough together. Cut the long cylinders into pieces about 1-inch long and crimp with your fingertips into gnocchi dumplings which are roughly marquise-shaped.
Bring a pot of water to boil for the gnocchi.
For the sauce:
Melt butter in a saucepan which will be large enough to accommodate the pound of cooked gnocchi, and saute onion until it becomes translucent. Mix in the flour to make a roux. Gradually add milk and allow to thicken. Stir in tomato sauce and heat through. Season with salt and pepper. Remove from heat and add cream cheese. Blend with an immersion blender. Immediately cover surface of sauce with a sheet of plastic wrap and set aside briefly.
Boil the gnocchi until al dente, drain, transfer to a serving dish, and spoon sauce over. Sprinkle with chives.