Heat your oven to 140°C
Heat your dutch oven over medium high heat and add some cooking oil.
Add in the lamb in batches, cooking until browned. Remove and set aside.
Slice the onions into julienned strips and add to the pot, stirring to lift the fond from the pan. Cook for 3-4 minutes, until softened and going golden.
Roughly chop your carrots into quartered pieces, grate your garlic and finely chop your rosemary. Add to the pan and cook for 2 minutes.
Add the lamb back into the pot. Sprinkle the flour over and stir well. Cook for a further 2 minutes.
Add the stock and worcestershire sauce and stir. Bring to the boil and then let simmer.
Peel the potatoes and slice in the mandolin. Melt half your butter.
Take the stew off the heat and layer the potatoes on top on concentric circles, starting from the outside and working in. Keep building up the layers until you run out of potatoes. Brush each layer with melted butter before adding the next one.
Put the lid on the pot and put it into the oven for 1 hour 30 minutes, keeping covered the whole time.
When the pot is ready to come out, remove the leaves from the thyme and melt into the remaining butter.
Remove the pot from the oven and take off the lid. Brush the thyme butter over the top.
Turn the oven up to 200°C and put the pot back in uncovered. Let cook for 30 more minutes to crisp and caramelise the top.