Add the beef into a non-stick pan and fry for 5 minutes until brown. Set aside.
In another pan, add 1 tbsp of olive oil and add onion, carrot, celery and leek. Cook on low//medium heat until soft for 4 minutes.
Add in beef, garlic, tomato puree, sun-dried tomatoes, herbs, salt and pepper. Fry for 3 more minutes.
Add the stock into the pan and simmer on medium heat, uncovered, for 25 minutes.
Once cooked, remove the bay leaves and thyme stalks.
In a large pot over medium heat, boil potatoes, cauliflower and celery for 12 minutes.
Drain the vegetables and blend in a food processor until smooth. Then, slowly add milk until desired consistency achieved.
Spread the beef mince and vegetables into a casserole dish, smooth out to the dish's edges and top with the mash.
Level the mash with a spatula and brush some olive oil on top.
Bake in a preheated oven at 190°C for 25 minutes.