Heat the oven to 400°F. Steam or boil the cauliflower florets until very tender, 10 to 12 minutes. Drain well and let them steam dry in the colander for 5 minutes.

Blend the cauliflower with the cream cheese, butter, half of the cheddar, and half of the salt in a food processor until smooth. Set aside.

Dice the onion, chop the mushrooms, and mince the garlic.

In a large oven-safe skillet over medium-high heat, brown the ground lamb, breaking it up, 8 to 10 minutes. Spoon off all but about 1 tablespoon of the fat.

Add the onion and mushrooms and cook until softened, about 5 minutes. Stir in the garlic and tomato paste and cook 1 minute.

Add the beef broth, thyme, rosemary, the remaining salt, and the pepper. Simmer 5 to 8 minutes, until the liquid has thickened into a gravy.

Spread the cauliflower mash over the filling, rough it up with a fork, and sprinkle with the remaining cheddar. Bake 20 minutes, until bubbling and golden. Rest 10 minutes before serving.