Heat oil in a heavy pot over medium-high heat and add carrots, celery, onion, and thyme. Cook, stirring occasionally, until vegetables start to soften, about 10 minutes.
Add flour and tomato paste and stir for 1 minute.
Add beef and cook until browned. Season to taste with salt and pepper.
Mix 1 cup of cheddar into mashed potatoes. Put the beef mixture into a large casserole and spread mashed potatoes over the top of it. Scatter 1/2 cup cheese over the top.
Bake until golden and bubbling, about 15-20 minutes. Let it cool slightly and then serve!
Notes
Ground Beef: We recommend an 80/20 fat version for the best flavor, but you can use a lower fat variety if you like.
Mashed Potatoes: You can make your own (I love my friend Ree’s recipe for mashed potatoes) or you can use the pre-made kind – look for them in the dair section of your supermarket.
Cheese: You can use anything from mild to extra sharp cheddar – pick your fave!
Onion: We like a sweet onion for this recipe, but any onion you have on hand will work.