Preheat your oven to 375°F (190°C).
Begin by heating the olive oil in a large skillet over medium heat. Add the diced onions, garlic, carrots, and celery, sautéing until the vegetables are softened, about 5-7 minutes.
Increase the heat to medium-high and add the ground beef to the skillet. Cook until browned, breaking it up with a spatula as it cooks.
Stir in the tomato paste, Worcestershire sauce, dried thyme, and beef broth. Bring the mixture to a simmer, then reduce heat and let it simmer uncovered for about 10 minutes, or until the liquid reduces slightly. Season with salt and pepper to taste.
While the beef mixture is simmering, steam the cauliflower florets until tender, about 6-8 minutes.
Once the cauliflower is cooked, transfer it to a food processor and pulse until it reaches a rice-like consistency.
In a mixing bowl, combine the riced cauliflower, shredded cheddar cheese, egg, garlic powder, and a pinch of salt and pepper. Mix until well combined.
Transfer the beef mixture into a 9-inch pie dish or an equivalent sized baking dish, spreading it out evenly.
Spread the cauliflower mixture over the beef layer, using a fork to create a textured surface that will brown nicely.
Place the dish in the oven and bake for 20-25 minutes, or until the cauliflower crust is golden brown and the edges are bubbling.
Let the cottage pie sit for a few minutes before serving.