Cut eggs in half lengthwise and pop yolks into a food processor; reserve whites. Add yogurt and salt to food processor and whirl until smooth, scraping down bowl as needed.
Transfer yolk mixture to a medium bowl and stir in mustard, 1 tbsp. chives, and 2 tbsp. onion to blend. Set egg whites on a platter, hollow side up. Scoop yolk mixture into a piping bag fitted with a star tip and pipe mixture into hollows. Sprinkle with more chives and onion if you like.
*For perfect hard-cooked eggs, cover them in cold water in a small saucepan and bring to a boil. Remove from heat and let stand 12 minutes. Plunge cooked eggs into ice water, crack all over, and let sit in the water for a few minutes before peeling. Look for spring onions (regular onions picked young) at farmers' markets in spring and at Latino markets year-round. They have a fatter bulb than green onions--which have less crunch but make a good substitute.
Make It Your Own
Try these variations on the above eggs; just sub in for the Dijon, chives, and spring onion.
PESTO: Substitute 3 to 5 tbsp. pesto. Garnish with tiny basil leaves.
HONEY-MUSTARD: Substitute 2 tbsp. honey mustard, 2 tbsp. chopped fresh dill, and 1 tsp. lemon juice. Garnish with rosemary blossoms.
CAPER-HOT PAPRIKA: Substitute 2 tbsp. chopped capers, 1/2 tsp. hot paprika, and 2 tbsp. chopped parsley. Garnish with parsley leaves, whole capers, and a dusting of hot paprika.