Turn your broiler to high heat.
Finely dice the celery. Peel and halve the carrot. Remove the thyme stems and chop the leaves.
Fill a large saucepan with water, add a pinch of salt, and bring to a boil. Add the potatoes to the saucepan.
Cook until tender, which should take about 20 minutes.
When done, mash the potatoes with butter, thyme, and sour cream. Mash until the texture is smooth.
Season with salt and pepper, and set aside.
Heat some oil in a large skillet. Add the vegetables; carrot, celery, and onion.
Cook until tender, and season with salt and pepper.
Add garlic powder and thyme, and cook for another minute.
Add beef on top of the vegetables, breaking it into pieces.
Cook until the meat is browned. Make sure both sides are cooked equally.
Add flour and stir occasionally, combining well.
Add tomato paste, water, and beef stock. Bring to a boil and cook until the mixture thickens.
Place the mashed potatoes on top.
Top with cheddar cheese.