DO-AHEAD TIP: Steam cauliflower.
Preheat oven to 350 degrees.
Heat the olive oil in a large skillet over medium-high heat. Add onion, carrot, celery, and garlic; cook and stir for 1 minute or until onions start to become translucent. Add ground sirloin and cook for 4 to 7 minutes or until browned and cooked through.
Transfer mixture to a large (and deep) baking dish; add next 9 ingredients (tomatoes through rosemary); stir until well blended and somewhat thickened.
In a large saucepan, combine remaining ingredients for Faux-Tay-Toes (cauliflower through pepper); mash together until fully blended.
Evenly spread cauliflower mixture over meat/tomatoes mixture in baking dish; bake for 25 minutes or until cauliflower turns golden brown on top.