Preheat oven to 350 degrees Fahrenheit. Butter a 2-quart casserole or gratin dish.

Layer the rounds of sliced potatoes in the buttered dish. Sprinkle with 1/2 teaspoon of salt and a few grinds of pepper.

Melt 3 tablespoons butter in a large skillet over medium heat. Add the leeks, 1 minced garlic clove, and the leaves from the sprigs of thyme. Saute for 5 minutes or until the leeks are tender and begin to turn golden.

Spread the leek mixture evenly over the potato rounds in the dish.

Add the cream, remaining garlic, bay leaf, and 1/2 teaspoon salt to the skillet. Simmer over medium-low heat for 5 minutes.

Pour the cream mixture evenly over the potatoes and leeks. Top with the grated cheese.

Cover with foil and bake for 30 minutes. Remove foil and bake for an additional 15 to 20 minutes, or until the cheese is bubbly and beginning to brown and the potatoes are tender.

Garnish with fresh thyme if desired and serve.