Vegetarian Shepherd's Pie Freezer Meal

In a large saute pan, heat oil over medium heat. Add carrots and onion and sauté until starting to soften. Add mushrooms and continue to cook until softened.
Sprinkle vegetable mixture with flour and stir to combine. Cook 1 minute. Add broth, tomato paste, thyme, rosemary, Worcestershire sauce, salt, and pepper. Stir to combine and simmer for 8-10 minutes until sauce thickens.
Turn off heat and stir in peas and corn. Pour mixture into a disposable baking dish and set aside.
Meanwhile, prepare the potatoes. Peel and quarter the potatoes and place in the instant pot. Add water until potatoes are just covered. Cook on steam setting for 12 minutes. Quick release the pressure, then drain the water. Add milk, butter, and salt and mash potatoes with the back of a wooden spoon.
Top vegetable mixture with the mashed potatoes, then sprinkle with cheese. Let cool completely. Cover tightly with plastic wrap and aluminum foil to protect from freezer burn.
When ready to cook, let thaw in the fridge if possible (remove plastic wrap and foil), then bake at 350 degrees for 30-40 minutes until bubbling and cheese is melted. If cooking from frozen, cooking time will be doubled.