Preheat oven to 400°F (200°C).
In a skillet, melt 15 ml (1 tablespoon) of butter over medium heat. Sauté the leeks, carrots, and mushrooms for 3 to 4 minutes.
Add the veal and the tomatoes, if desired. Season with salt and pepper. Simmer over medium-low heat for 10 to 12 minutes, until the liquid has evaporated. Transfer the mixture to a 20 cm (8 inches) baking dish.
Meanwhile, with a mandolin or a grater, slice the potatoes into fine julienne. Pat dry with paper towels. In a bowl, toss the potatoes with the remaining butter and the thyme, if desired. Season with salt.
Top the veal mixture with the julienned potatoes. Bake for 20 to 25 minutes, or until the potatoes are tender and golden.