Place the cauliflower in a steam basket, set it over salted boiling water and cook until tender, about 8 to 10 minutes.
Meanwhile, put the rutabaga, carrots and green beans in a microwave safe bowl. Add about a quarter of a cup of water and a generous pinch of salt, cover loosely and cook in the microwave on high for 3-4 minutes, until just slightly softened (you only want to blanch the vegetables, not cook them all the way through)
Transfer the cooked cauliflower into your blender; add chicken stock, ghee, Dijon mustard, thyme, salt, pepper and nutmeg and process on high speed until super smooth and silky in consistency. Set aside.
Put about half the beef tongue meat at the bottom of a large casserole dish. Top with about half the veggies and one third of the cheese. Repeat with rest of the meat, veggies and another third of the cheese then pour the cauliflower béchamel right over the casserole. Spread it evenly across the dish and sprinkle the remainder of the cheese all over the top.
Bake in a 375F oven for about 30 minutes or until the béchamel starts to bubble and the cheese turns golden brown.
Take the casserole out of the oven and let it rest for 10-15 minutes before serving.
Garnish with a drizzle of extra-virgin olive oil and fresh thyme if desired.