Preheat oven to 300°.
To prepare pot roast, melt butter in a large Dutch oven over medium-high heat. Add onion to pan; sauté 12 minutes or until almost tender. Reduce heat to medium-low; cook for 40 minutes or until onions are caramelized, stirring frequently. Transfer onions to a bowl.
Place pan over medium-high heat. Coat pan with cooking spray. Sprinkle roast evenly with 1 1/2 teaspoons salt and 1/2 teaspoon pepper. Add roast to pan; cook 5 minutes, turning to brown on all sides. Add onion mixture, broth, thyme, and beer to pan; bring to a simmer. Cover and bake at 300° for 2 hours or until tender, turning over halfway during cooking time.
Remove roast from pan. Cover and keep warm. Place pan over medium-high heat. Add cornstarch to pan, stirring with a whisk; bring to a boil. Cook for 1 minute, stirring constantly.
To prepare potatoes, place fat-free milk and potatoes in a large saucepan; bring to a boil. Reduce heat, and simmer 15 minutes or until tender. Mash potatoes; stir in poppy seeds, 3/4 teaspoon salt, and 1/2 teaspoon black pepper.
Serve roast with potatoes and sauce.