Cheesy Potato Gratin

INGREDIENTS

• 1 Tbs. unsalted butter
• 1 shallot, finely chopped
• 2 Tbs. all-purpose flour
• 1/2 cup chicken stock
• 1/2 cup heavy cream
• 1 cup grated Gruyère cheese
• 1 tsp. chopped fresh thyme
• Salt and freshly ground pepper, to taste
• 3 lb. Yukon Gold potatoes, peeled and cut into 1 1/2-inch chunks
• 1/2 cup grated Parmigiano-Reggiano cheese