In a large pan over medium heat, heat the olive oil.

Sauté the onion and garlic until the onion is slightly translucent, about 5 minutes.

Add the celery and carrots, and cook for 5 more minutes.

Add the ground beef, and season with salt and pepper. Allow to brown, about 5 minutes, and then add the tomato paste, mustard, thyme, and rosemary.

Cook for about 10 more minutes, or until any liquid in the pan begins to evaporate.

Add the chicken broth, cook for 5 to 7 minutes to reduce it a bit, and then add the peas. Give it a quick stir, and transfer the mixture into one baking dish or individual ramekins.

While the filling is cooking, fill a large saucepan with water and bring it to a boil.

Add the cauliflower to the boiling water, and cook until it is fork-tender, about 10 minutes.

Drain the cauliflower and return to the pan.

Add the butter and garlic powder, and season with salt and pepper. Use a mixer or immersion blender to mash the cauliflower until it is mostly smooth. If it is too thick, add some coconut milk.

Spread the mashed cauliflower evenly over the beef, and brush the top with the additional ½ tablespoon of melted butter.

Place under the broiler for about 5 minutes, or until the mashed cauliflower becomes golden-brown.

Serve garnished with the scallions.