Melt butter in a large Dutch oven over med high. Add onion; saute 12 min or til almost tender. Reduce heat to medium low; cook 40 min or til onions are caramelized, stirring frequently. Transfer onions to a bowl.
Place pan over med-high. Coat pan with cooking spray. Sprinkle roast evenly with 1 1/2 t salt and 1/2 t pepper. Add roast to pan; cook 5 min, turning to brown on all sides. Add onions, broth, thyme, and beer to pan; bring to a simmer. Cover and bake at 350 for 2 hours or til tender, turning over halfway during cooking time.
Remove roast from pan. Cover and keep warm. Place pan over med high. Add cornstarch to pan, stirring with a whisk; bring to a boil. Cook 1 min, stirring constantly.
To prepare potatoes, place fat free milk and potatoes in a large saucepan; bring to a boil. Reduce heat and simmer 15 min or til tender. Mash potatoes; stir in poppy seeds and remaining salt and pepper.