Preheat the oven to 450°F.
Place the bread slices in a large mixing bowl.
Pour the milk over the bread.
Allow the bread to soak for 3 to 4 minutes, or until softened.
Add the eggs, horseradish, ground beef, salt, and black pepper to the bowl.
Mix until the ingredients are evenly combined.
Form the mixture into small, evenly sized meatballs.
Arrange the meatballs in a single layer on a nonstick, rimmed baking sheet.
Place the baking sheet in the preheated oven.
Bake the meatballs for 10 minutes.
While the meatballs are baking, prepare the sauce.
In a separate bowl, combine the ketchup, pure maple syrup, soy sauce, Dijon mustard, and water.
Stir until the sauce is smooth and fully combined.
Remove the meatballs from the oven after the initial baking time.
Carefully drain any excess grease from the baking sheet, if necessary.
Pour the prepared maple mustard sauce over the meatballs.
Turn or toss the meatballs gently until they are evenly coated with the sauce.
Return the baking sheet to the oven.
Bake for an additional 15 minutes, or until the meatballs are fully cooked.
Verify that the internal temperature of the meatballs reaches 160°F using an instant-read thermometer.
Remove the meatballs from the oven.
Garnish with chopped green onions or parsley, if desired.
Serve warm.