1Heat oven to 375°F. Spray shallow 2-quart casserole with cooking spray. In 12-inch nonstick skillet, cook beef, onion, carrots and garlic over medium-high heat 5 to 7 minutes, stirring occasionally, until beef is thoroughly cooked; drain. Stir in broth, tomato paste, Worcestershire sauce, 3/4 teaspoon of the salt and the thyme. Heat to boiling; reduce heat. Cover; simmer 10 minutes.
2In small bowl, mix flour and water. Stir into beef mixture. Cook 2 minutes, stirring until thickened. Stir in spinach. In large saucepan, place potatoes and enough water to cover. Simmer 10 to 15 minutes or until tender; drain. In large bowl, beat potatoes, milk, butter and remaining 1/2 teaspoon salt until smooth.
3Spoon beef mixture into casserole. Top with potato mixture. Bake uncovered 30 minutes or until potatoes begin to brown.