To make the mashed potatoes, boil the potatoes until soft, then mash with butter, milk, mustard, salt, and pepper.
Cook leeks in vegetable stock until soft.
Preheat oven to 210C/190C fan.
Slice brussels sprouts, pick tarragon leaves.
Add butter, tarragon, brussels sprouts, garlic, salt, and pepper to the pan with leeks.
Whisk stock with cornflour, then add to the pan along with peas, nutmeg, and quark.
Transfer the filling to a pie dish, top with mashed potatoes and grated cheddar.
Bake in the oven for 15 minutes.
Sprinkle chopped tarragon over the top before serving.