Preheat oven to 350 degrees. Wash and dry all produce. Chop sweet potatoes in small cubes. Put in a pot and cover sweet potatoes with water. Bring to a boil.
In a large pan over medium high heat, add oil and ground beef. Cook meat until no longer pink, about 7-8 minutes.
While meat is cooking, chop celery, garlic and scallions (separating white and green parts). Grate carrots with vegetable grater. When meat is cooked, drain grease from pan. Put cooked meat in large bowl.
Return pan to the stove over medium high heat. Add scallions, carrots, celery, garlic, thyme. Cook for 10 minutes, or until soft. Check the sweet potatoes; if you can poke them easily with a fork, they are done.
When sweet potatoes are done, drain the water and return them to the pot. Mash sweet potatoes with butter, salt, and pepper.
When the veggies are cooked, mix with ground beef and evenly spread in casserole dish. Top meat and veggies with mashed sweet potatoes, covering all the meat. Bake for 15 minutes, or until potatoes start to brown. Top with scallion greens before serving.