Brown the ground beef in a large skillet, in batches if needed. Drain off any fat and add cooked ground beef to a slow cooker.
Add the onions, mixed vegetables, garlic, flour, tomato paste, beef stock, Worcestershire sauce, and thyme to the slow cooker.
Cover and cook on high for 4-5 hours.
To make the mash:
Meanwhile, make the mashed potatoes. Boil potatoes in water until tender which usually takes between 16-20 minutes.
Drain the potatoes well. Add butter and milk and use a potato masher to mash them very well. Season with salt and pepper, add the cheese, and mix well.
To make the cottage pie:
Spoon the meat from the slow cooker into a large casserole dish (about a 9 by 13 in size).
Spread the cheesy mashed potatoes over the beef.. Top with additional shredded white cheddar if desired.
Bake at 425 degrees F for 25-30 minutes until the mashed potatoes are slightly browned on top.
Make this with ground lamb to make shepherd’s pie or you can also make this with ground turkey or ground chicken if you would like. You can also use leftover roast beef like from a Sunday roast.If you need to save even more time, use leftover mashed potatoes or even boxed potatoes and just add the cheese.Swap out the mixed veggies for fresh cut veggies but you'll want to sauté them before adding to the slow cooker. Some ideas, use parsnips or a mix of root vegetables instead of just the carrot and add or swap peas for the corn.You can easily make this ahead or make a double batch and freeze one of the casseroles for later. Use a foil pan for one of the batches and cover with foil before placing in the freezer. Don't forget to put the date on it! It will keep for about 3 months in the freezer.