Preheat oven to 200°C/390°F. Wash and slice leek, then set aside. Peel potatoes and cut into bite-sized pieces. Add cut potatoes to a large pot and fill up with water until they are covered. Salt water, then bring to a boil. Reduce the heat and cook for approx. 10 min. over medium heat.
Add milk and heavy cream to a small pot and bring to a boil. Reduce heat to medium, then add nutmeg and thyme. Let simmer for approx. 10 min., or until thickened. Season with salt and pepper to taste.
Slice vegetarian sausages. Heat oil in a frying pan and fry slices for approx. 4 – 5 min.
Grease baking dish with butter. Add precooked potatoes, fried sliced vegetarian sausages, sliced leek, and sauce to the baking dish and stir to combine. Grate cheddar and Gruyère cheese on top and sprinkle breadcrumbs over the cheese layer. Bake in the oven at 200°C/390°F for approx. 50 – 60 min., or until golden brown. Remove from the oven and let rest for approx. 5 – 10 min. before serving. Enjoy!