Preheat oven to 400 degrees.
Place cut sweet potatoes on a large baking tray and coat with 2 tablespoons of olive oil. Sprinkle with salt and pepper.
Roast for about 40 minutes until they are slightly browned and tender (if they start to burn cover them loosely with foil).
Keep oven on and transfer sweet potatoes to a bowl and mash.
Meanwhile, in a separate pan, sauté the ground beef and onion (unless the ground beef is very lean you do not need to add oil) over medium heat until beef is cooked and onions are translucent.
Add the 2 teaspoons of fresh thyme (1 teaspoon if dried) and 2 teaspoons of salt along with the ground pepper. Turn off heat.
In another pan with a lid, cook the vegetables with the broth covered over medium heat until tender, about 20 minutes.Then strain.
In a rectangular 13" pan, layer the vegetables on the bottom, then the ground beef , and top with the mashed sweet potatoes.
Drizzle a tablespoon of melted butter on top of the potatoes.
Bake for 20 minutes.