Beefy Banger Mash Pies with Onion Gravy

Name: Beefy Banger Mash Pies with Onion Gravy
Ingredients: 1 tbsp (15 ml) olive oil; 1 large (approx. 200g) yellow onion, finely chopped; 2 cloves garlic, minced; 1 lb (454g) ground beef (80/20 blend recommended); 1 tsp dried thyme; 1/2 tsp dried rosemary; 1/4 tsp ground nutmeg; 1/4 cup (60 ml) all-purpose flour; 1 cup (240 ml) beef broth; 1/2 cup (120 ml) Worcestershire sauce; 1/4 cup (60ml) tomato paste; Salt and freshly ground black pepper to taste; 2 lbs (907g) russet potatoes, peeled and quartered; 1/2 cup (120 ml) whole milk, warmed; 4 tbsp (56g) unsalted butter, softened; Salt and freshly ground white pepper to taste; Optional: 1/4 cup grated parmesan cheese; 2 tbsp (30ml) unsalted butter; 2 large (approx. 400g) yellow onions, thinly sliced; 1 tsp sugar; 2 cups (480 ml) beef broth; 2 tbsp (30ml) cornstarch mixed with 2 tbsp cold water (slurry); 1 tbsp balsamic vinegar