Melt remaining 2 tablespoons of butter in a small saucepan on medium-high and stir in the scallions and garlic. Sauté for 2 minutes. Add 2 tablespoons of flour and cook for 2 more minutes. Now that you have made your roux, add the milk, nutmeg, salt and pepper and continue cooking and stirring until thick and bubbly. Remove from heat, add kale, cheddar cheese and the remaining 2 tablespoons of Parmigiano-reggiano cheese. Place in large bowl to cool for a few minutes, then stir in the egg yolks.