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