Heat the oil in a medium-sized skillet over medium heat. Add the chopped onion and saute, stirring occasionally, until onion is transluscent, about 8 minutes. Add the chopped garlic and continue sauteing another 3 minutes.

While the onion is sauteeing, steam the cauliflower for 8 minutes, or until it's nice and tender. Drain the cauliflower and transfer it to a blender along with the sauteed onion and garlic.

Add the broth, ghee, nutmeg, sea salt, and thyme to the blender and blend until completely creamy.

Preheat the oven to 400 degrees F and lightly oil a large (9" x 13") casserole dish.

Slice the sweet potatoes into thin rounds (about 1/8-inch thick). 

Layer 1/3 of the sweet potato slices in the casserole dish and pour in 1/3 of the sauce. Spread the sauce evenly over the sweet potato slices. Repeat for remaining sweet potatoes and sauce.

Cover casserole dish with foil. Place casserole dish on the center rack of the preheated oven and bake for 40 minutes. Remove foil and bake another 30 to 40 minutes, or until sauce is bubbly and sweet potatoes are cooked through.

Allow casserole to rest 15 minutes before slicing and serving.