In a large sauté pan, melt 1 tbsp. butter over medium heat; cook onions until soft (about 5 min.). Stir in flour to coat onions; cook about 1 min. Gradually whisk milk and cream into onion mixture, stirring constantly; simmer sauce 1 min. Stir in Boursin cheese a little at a time until melted; remove pan from heat. Stir in spinach, Parmesan, lemon zest, salt and pepper. Divide mixture among 4 (4 oz.) greased ramekins. Microwave remaining butter in a small bowl until melted; stir in crumbs. Top spinach mixture with crumbs. Bake ramekins in a preheated 425° oven on a jelly-roll pan until crumbs are golden-brown (20-25 min.); serve immediately.