Preheat oven to 350 degrees F. Cook beef in a pot over medium heat until no longer pink. Drain fat, then add onions, carrot, garlic and thyme and cook five minutes. Add tomato sauce, peas and corn and heat through. Season mixture with salt and pepper and then divide among 2, 9 by 13 inch, or a series of smaller, casseroles.
Top with mashed potatoes. (If freezing for later use, do so now, but first cool shepherd’s pie to room temperature. Thaw in fridge overnight before baking as described below). Bake 50-60 minutes, until potatoes are golden and filling is bubbling. Serve with a simple green salad and warm bread or rolls.