About 3/4 cup stale white breadcrumbs or panko breadcrumbs
⅓ cup half-and-half or whole milk
1 lb. ground sirloin (90% lean)
Kosher salt and coarsely ground black pepper
¼ cup (a small handful) fresh flat-leaf parsley leaves, finely chopped
About 3 tbsp. grated onion with juices
2 tbsp. Worcestershire sauce
1 large egg yolk
1 large clove garlic, grated or finely chopped
Cooking spray for spraying, or vegetable oil for brushing
1 cup sour cream or Greek yogurt
¼ cup finely chopped fresh chives
3 tbsp. beet horseradish (such as Gold’s)
Preheat the oven to 425°. Line a baking sheet with parchment paper.
In a medium bowl, mix the breadcrumbs and half-and-half. Let stand until the crumbs soften, 3 to 5 minutes. Add the beef. Season with salt and pepper. Mix in the parsley, onion with juices, Worcestershire, egg yolk, and garlic. Roll the mixture into balls about the size of a large walnut. Arrange on the baking sheet; spray with cooking spray or brush with oil. Roast the meatballs, turning once, until browned and just cooked through, about 18 minutes.
While the meatballs are roasting, in a medium bowl, mix the sour cream, chives, and beet horseradish. Season the sauce.
Serve the meatballs with the sauce for dipping or topping.