In a large skillet, add half of the stick of butter and heat over medium heat. Add chopped onions and cook on medium heat until onions are translucent. Add garlic and cook for another minute. Sprinkle with half the salt at this point. Add spinach and artichokes to skillet and add remaining salt and all of pepper. Cook for 4 to 5 minutes. In a separate large pot, add the other half stick of butter. Melt butter over medium heat. Add flour to butter and cook for 1 minute, making a roux. To this add chicken stock and whisk together until it is completely smooth. Turn heat up to medium-high and add heavy cream; cook until it just begins to boil. Reduce heat. Allow to boil for 1 minute and add cheeses. Cook about 2 more minutes and add spinach mixture from skillet. Serve with tortilla chips or crostini.