Preheat oven to 190°C (375°F). Grease 9 x 13-in. (3-L) baking dish. Melt 3 tbsp (45 mL) butter in saucepan over medium heat. Add garlic and thyme; cook 1 to 2 min. until softened. Sprinkle in flour. Cook, stirring, 2 to 3 min. until flour absorbs butter. Gradually whisk in milk until smooth. Continue whisking sauce while bringing just to a boil. Reduce heat to medium-low. Cook 6 to 7 min. until slightly thickened. Remove from heat. Stir in half the cheese, plus the mustard, salt and pepper.
Layer half the potato slices and half the sliced onions in baking dish. Pour half the sauce evenly over top. Layer with remaining onions and potatoes, being sure to end with potatoes on top. Pour remaining sauce over top. Sprinkle with remaining cheese and dot with remaining butter. Arrange sprigs of thyme over top. Cover tightly with foil; place baking dish on baking sheet.
Bake 1 hr. Remove foil. Bake additional 30 min. until potatoes are tender and top is golden brown. Let stand 10 min. before serving.
Add a pinch of nutmeg to the cream sauce as it cooks for a hint of extra flavour.