To make your own bread crumbs, roughly cube a couple slices of bread and blitz in a food processor until a small crumb. Season with salt & pepper and paprika (optional). Transfer to a bowl and set aside.
To prep to gratin: have a bowl of cold water with lemon juice ready before you start prepping the burdock. Peel and cut the burdock into pieces (about 1 ½ inches) and add to the bowl of water (this stops oxidizing).
Clean the leeks, and cut in half down the middle and then cut in half or thirds.
Add the burdock pieces and leeks to a baking dish in layers.
Season with thyme, salt and pepper. Pour milk or cream into the baking dish until it’s about ½ way up the veggies. Sprinkle with cheese and pop into a 350F oven until it’s bubbling and the veggies are soft (about 30 mins).
Remove from the oven, cover with more cheese (optional) and the bread crumbs and return to the oven until the topping is brown and crunchy (about 15-20 mins).