Preheat oven to 375 degrees F. Heat a 12-inch ovenproof skillet over medium-high heat. Add beef, onion and garlic; cook, stirring often to crumble, until the beef is browned and onion is tender, 8 to 10 minutes. Stir in drained tomatoes, Worcestershire, rosemary, 1 tablespoon parsley, 1 teaspoon pepper and 1/4 teaspoon salt. Whisk broth and flour in a small bowl; stir into the mixture in the skillet. Cook over medium-low heat, stirring often, until the mixture thickens, about 15 minutes.
Meanwhile, place cauliflower and water in a large saucepan; bring to a boil over high heat. Cover and reduce heat to medium-high; cook until the cauliflower is tender when pierced with a fork, about 10 minutes. Drain. Add butter, cream and the remaining 1/2 teaspoon salt; mash with a fork or potato masher until smooth (you will have about 4 cups mashed).
Carefully spoon the mashed cauliflower over the hot mixture in the skillet, gently spreading it over the top (do not mix the 2 layers together). Sprinkle with the remaining 1/2 teaspoon pepper.
Bake until the top is beginning to brown and the filling is bubbly, 22 to 25 minutes. Sprinkle with the remaining 2 tablespoons parsley before serving.
Tips
To make ahead: Follow recipe as directed through Step 2. Refrigerate the beef mixture and the mashed cauliflower topping in separate airtight containers for up to 4 days. To reheat, return beef mixture to skillet, and heat over medium heat until hot. Meanwhile, microwave cauliflower topping on Medium until just warmed, about 1 minute. Proceed with Steps 3 and 4.
