Place eggs in a pot of cold water, bring to a boil, then remove from heat and let sit covered for 10-12 minutes until fully hard-boiled. Transfer to an ice bath to cool completely, about 5 minutes. Once cooled, gently crack and peel each egg under cool running water, starting from the wider end where the air pocket is. Pat dry with paper towels.

Carefully slice each cooled egg in half lengthwise with a sharp knife, creating two even halves. Use a small spoon to scoop out the yolks into a mixing bowl, being careful not to damage the delicate egg white cups. Set the hollow egg white halves aside on a serving plate.

Add the mayo and sour cream to the bowl with yolks and mix with a fork or whisk until completely smooth and creamy—I like using a fork rather than a mixer to keep things simple and avoid over-working the filling. The mixture should be thick enough to pipe but light and fluffy. Stir in the Dijon mustard to add a subtle tang that brightens the richness of the mayo and sour cream.

Fold in the minced chives, salt, black pepper, and onion powder into the yolk mixture until evenly distributed. Taste and adjust seasoning as needed—I find the chives add a nice fresh bite that balances the richness, so don't skip them. The filling should be flavorful and ready to pipe.

Transfer the filling mixture into a piping bag (or a zip-top bag with a small corner cut off for a simpler approach). Pipe the filling generously into each egg white half, creating an attractive mound. Arrange the filled deviled eggs on a serving platter and chill until ready to serve if making ahead. They're best served cold or at room temperature within a few hours of assembly.