Heat oil in a frying pan over medium heat. Add onion, garlic, carrot and celery. Cook, stirring, for 10 minutes or until softened. Add rosemary. Cook for 1 minute or until fragrant. Increase heat to high. Add mince. Cook, stirring with a wooden spoon to break up mince, for 6 to 8 minutes or until browned.
Add flour. Cook, stirring for 2 minutes. Add worcestershire sauce and paste. Cook, stirring, for 1 minute. Add stock. Bring to the boil. Reduce heat to low. Cook, stirring occasionally, for 25 to 30 minutes or until thickened. Season with salt and pepper.
Meanwhile, place potato in a large saucepan. Cover with cold water. Bring to the boil over high heat. Cook for 10 to 12 minutes or until tender. Drain. Return to pan over low heat. Add milk and ½ the butter. Mash until smooth.
Preheat oven to 200C/180C fan-forced. Spoon mince mixture into an 8 cup-capacity baking dish. Top with mash, using a fork to spread out over mince mixture. Dot with remaining butter. Bake for 20 minutes or until golden. Serve.