Mix minced meat with breadcrumbs, egg, pressed garlic, mustard, salt, and pepper. Use a tablespoon to form equal-sized balls.
Heat oil in a non-stick pan over medium heat. Fry meatballs for 4 minutes until golden brown. Drain on kitchen paper.
Cut red onions into eighths and place in the same pan.
Cook onions over medium heat until golden brown. Deglaze with white wine, then add chicken broth. Cook uncovered for 10 minutes until liquid is reduced by half.
Mix sour cream thoroughly, then stir it quickly into the sauce to prevent curdling.
Add meatballs back to the pan with the herb mixture. Simmer over medium-low heat for 10 minutes until sauce is creamy and meatballs are cooked through.