Place meat in a bowl. Grate onion using a box or handheld grater directly over the meat bowl. Add sage, egg, bread crumbs, Worcestershire and mustard, salt and pepper then mix meat to combine. Score meat into 4 sections and form 4 oval shaped loaves, 1 to 1/2 inches thick.
Preheat 2 tablespoons extra-virgin olive oil, 2 turns of the pan, in a large nonstick skillet over medium-high heat. Add the meatloaves and cook 6 to 7 minutes on each side. Remove to a platter and cover with foil to keep warm.
While meatloaf cooks, add chicken stock and 1 tablespoon of extra-virgin olive oil, 1 turn of the pan, to a deep pot. Trim away greens and stems from the base of cauliflower. Cut away the core with a sharp paring knife. Set cored head into pot with broth. Cover pot and bring liquid to a boil. Reduce heat to medium low and cook cauliflower, covered, until tender, 8 to 10 minutes. Uncover the pot and mash the cauliflower. Let some liquid cook off as you smash the cheeses into the cauliflower. As the mixture thickens, turn off heat and season with salt, pepper and nutmeg. Stir in chives and transfer cauliflower to as serving dish.
Once meatloaves are removed, place the skillet back over medium heat. Add extra-virgin olive oil, 1 turn of the pan, and the mushrooms. Cover the skillet with foil and nest the foil into pan with a second smaller skillet, smothering the mushrooms. Cook until deep brown and tender, 7 to 8 minutes. Remove the foil, add flour and cook a minute longer. Add in a shot of Scotch or sherry, if you have either on hand, then whisk in the beef stock. Stir in half-and-half or cream and bring to a bubble. Season the mushroom gravy with salt and pepper, to taste.
Place the meatloaves on dinner plates and douse with hot, smothered mushroom gravy, spoon the cauliflower alongside.