Cut diced beef in half if too large. Heat oil in a large heavy-based saucepan over medium-high heat. Add beef and cook, stirring, for 4 minutes or until browned. Transfer to a plate. Reduce heat to medium. Add onion to pan. Cook, covered and stirring occasionally, for 10 minutes or until onion is softened and beginning to colour. Add sugar, garlic and three-quarters of the thyme. Cook, stirring, for 5 minutes or until fragrant and caramelised. Reserve ⅓ cup onion mixture. Return beef to pan. Add sachet and stir to coat.
Gradually add stock, stirring to combine. Bring to the boil. Reduce heat to medium-low. Cover and simmer, stirring occasionally, for 1 hour and 30 minutes or until beef is very tender. Remove from heat. Carefully divide beef mixture among 4 x 1 ½ -cup-capacity ramekins and place on a baking tray.
Preheat oven grill to medium-high heat. Top each ramekin with 2 bread slices. Spoon over a little reserved onion, then sprinkle with cheese. Grill for 4 minutes or until melted. Stand for 5 minutes. Serve.
NOTE: This is an Australian recipe. The brown onions are our standard bagged smallish brown onions which come in the 3 lb bags. A "punnet" is a small plastic container, so this would be of fresh thyme on a hanging-type container. Name brands aside, typically French Onion Soup is topped with Gruyere cheese.
