Preheat the oven to 350 degrees. Grease a 9 inch by 13 inch baking dish with butter. 

Melt half of the butter in a saute pan on medium-low heat. Add the diced onions to the pan with 1 teaspoon of salt and stir constantly until the onions are golden brown and caramelized, about 30 minutes.

Bring a large pot of salted water to a boil. Add the egg noodles into the pot and cook for 5 minutes. Drain and set aside.

Squeeze out any liquid from the defrosted spinach and set the spinach aside. 

Melt the remaining butter in a microwave or pan and set aside. 

Place the caramelized onions, cooked noodles, spinach, melted butter, remaining salt, sour cream and eggs into a large bowl. Gently mix until everything is combined. Transfer the kugel mixture into the baking dish and use a spatula to even out the surface.

Bake the kugel for 35 to 40 minutes or until the kugel is golden brown and cooked through.

Serve hot.