Filling: Heat butter in saucepan. Add onion. Saute until soft. Remove from heat.
Stir in next 5 ingredients.
Spoon 1/4 cup egg mixture down centre of each tortilla. Roll snugly. Place seam side down in baking dish large enough to hold single layer. Cover with damp cloth while rolling remainder to prevent cracking.
Sauce: Stir first 6 sauce ingredients together in bowl. Pour over enchiladas.
Sprinkle with cheese. Bake, uncovered, in 350 F oven for about 25 minutes until bubbly hot.
Spoon sour cream across enchiladas. Top with chives.