Take a large baking tray and line it with non-stick baking paper.
Spread the mince out on the baking paper and put on the top shelf of the oven
Bake for 45 minutes, taking it out to break up the pieces every 10 minutes or so
Meanwhile, dice the onion, carrot and celery finely
Heat the oil or dripping in a large heavy bottom pan
Stir through the onion, carrot and celery mixture and cook for 5 minutes or so till starting to soften. Stir through the herbs and tomato puree and continue to cook for a couple of minutes.
Once the mince is baked, add it to the onion, carrot and celery mix and add in the stock and wine. Cook for 30 minutes or so till the mixture has reduced a little and darkened in colour
Peel the potatoes and celeriac and cut in half if necessary.
Parboil for 5 minutes
Drain and cool for a few minutes before grating into a large bowl
Season with salt and pepper. Stir the butter or olive oil through the potato mixture, using a fork to separate it as much as possible.
Once the mince is cooked, stir through the lentils and taste. Season with salt and pepper and put into an ovenproof casserole
Top with the potato and celeriac rosti and bake in the oven at 175c for 30 minutes
You can make this dish up to the stage where you've topped it with potato and celeriac rosti. It will keep in the fridge for 3 days or can be frozen for up to 3 months. Defrost completely before cooking if you have frozen it.