Heat oven to 400°F; oil rimmed sheet. Gently mix pork, oats, apple, egg, sage, 1/2 teaspoon salt, and pepper just until combined.
Shape 24 equal meatballs; space on sheet. Bake 20 to 24 minutes until browned and centers 160°F.
Meanwhile, melt butter in wide saucepan over medium. Add onions and remaining salt; cook 15 to 18 minutes, stirring, until soft and lightly golden.
Stir in flour 1 minute. Gradually whisk in broth and juice; simmer 6 to 8 minutes until lightly thickened. Add mustard.
Add cooked meatballs; gently simmer 3 minutes. Divide among six shallow containers, four meatballs and equal gravy each. Refrigerate within 2 hours; chill fully.
Cover; freeze up to 3 months. Reheat one frozen portion with measured reheating water in small covered saucepan over low.
Turn meatballs and stir gravy as they thaw; gently simmer until every center reaches 165°F, usually 20 to 30 minutes. Add gravy to each bite to keep oats soft.