Preheat oven to 190 degrees C (375 degrees F). In 3-quart saucepan, melt Becel® Buttery Taste margarine over medium-high heat and cook onion, stirring occasionally, 5 minutes or until starting to brown. Add garlic and cook 30 seconds. Stir in flour and cook, stirring, 1 minute. Gradually whisk in milk, and bring to a boil. Add broth and return to a boil 1 minute. Add herbs and black pepper. Remove from heat and stir in 125 g (4 oz.) cheese; Set aside.
In 13x9-inch shallow casserole dish, lightly sprayed with nonstick spray, arrange of the potatoes in overlapping pattern. Stir cheese mixture and pour half over potatoes. Repeat with remaining potatoes, and cheese mixture.
Bake uncovered 45 minutes. Combine remaining cheese and breadcrumbs and sprinkle over potatoes. Bake additional 10 minutes or until potatoes are tender and top is golden brown.