Steps

In saucepan, bring cream, garlic, peppercorns, thyme and salt to boil; reduce heat to low, cover and simmer for 15 minutes. Remove from heat; set aside.

Meanwhile, in skillet, heat oil over medium-low heat; cook onion, stirring often, until light brown, about 45 minutes. Slice potatoes as thinly as possible (using mandoline if available).

Layer one-quarter of the potatoes in greased 8-inch (2 L) square glass baking or casserole dish; top with one-third of the onion and one-quarter of the cheese. Repeat twice. Top with remaining potatoes. Strain cream mixture over potatoes, shaking casserole to distribute evenly; sprinkle with remaining cheese.

Bake in 300°F (150°C) oven until tender and knife inserted in bottom pierces potatoes easily, 1-1/2 hours to 2 hours. (Make-ahead: Let cool; cover and refrigerate for up to 2 days. Reheat in 375°F/190°C oven for 30 minutes.)

Tip:
For a lighter version, steep garlic, peppercorns, thyme and salt in 2 cups (500 mL) milk. Then make béchamel sauce: In saucepan, melt 1 tbsp (15 mL) butter over medium-low heat. Add 2 tbsp (25 mL) all-purpose flour; cook, stirring, for 1 minute. Whisk in strained milk mixture; simmer for 5 minutes. Pour over potatoes.