Classic Scalloped Potatoes

WHAT YOU'LL NEED

• 1/4 cup I Can't Believe It's Not Butter!® Spread
• 1/2 cup chopped onion
• 1/4 cup all-purpose flour
• 2 cups whole milk
• 1 cup heavy cream
• 1/4 tsp. ground black pepper
• 2 fresh sage leaves or thyme sprigs (optional)
• 1 tub Knorr® Homestyle Stock - Chicken
• 2 cups shredded Swiss cheese (about 8 oz.)
• 4 lbs. Yukon gold potatoes, peeled and cut into 1/8-in. slices