Wash and peel the potatoes, then cut into cubes (about ¾ inch). Boil in salted water for 5 minutes or until just tender. Drain, then set aside.
Preheat oven to 375℉. Pierce pit pastry on bottom, and along sides, with a fork several times to vent the crust.
Sauté onion and leeks in butter over medium heat for about 10 minutes or until slightly caramelized. Add garlic, then continue cooking for 3 minutes. Remove from heat.
Add thyme, salt, pepper, and cream to onions, then mix well. Add potatoes and HALF of the cheese, then gently fold until well combined.
Spoon mixture into pie pastry, ten top with remaining cheese. Bake at 375℉ for 25-30 minutes or until cheese is just starting to brown.
Let pie rest for 10 minutes before serving. Garnish with fresh thyme if desired.