Sauté a diced onion in 1 tablespoon of olive oil with a sprinkle of salt on low heat for about 3-4 minutes.
Add diced mushrooms, garlic, and herbs, then turn up the heat to medium. Sauté until the mushrooms start to release their juices.
Add vegan beef crumbles (or your protein of choice) and cook for an additional 2-3 minutes.
Add 4 cups of frozen mixed veggies and saute for about 5 more minutes until they are thawed.
Sprinkle with 2 tablespoons of flour and mix until well combined.
Add 1 3/4 cups of vegetable broth, 2 tablespoons tomato paste, and 1 tablespoon soy sauce, then stir well.
Cook until the mixture starts to bubble and thicken, then pour it into a large casserole dish.
Make the vegan mashed potatoes
Fill a large pot about 1/3 full of cold water.
Wash and peel your potatoes, then cut them into cubes about 1 square inch in size.
Place the potato cubes in the pot of cold water as soon as you cut them. (This will keep them from turning brown.)
Add salt to the cooking water and bring the potatoes to a boil.
Adjust the heat so that the potatoes remain at a rolling boil for about 15-20 minutes, until they break easily when pierced with a fork.
Drain the cooking water and return the potatoes to the pot. Immediately add 2 tablespoons vegan butter, 3/4 cup plain plant-based milk, 1/2 teaspoon salt, and 1/2 teaspoon garlic powder to the hot potatoes.
Mash with a hand potato masher until the potatoes are fluffy.
Assemble and bake the shepherd's pie
Spread a thick layer of vegan mashed potatoes over top of the "meat" and veggie sauce, then drizzle with 2 tablespoons of melted butter (or brown butter sage sauce).
Bake at 350° F for 30 minutes. You can turn it to broil for the last 3-4 minutes to brown the top, if you prefer.
Garnish with fresh thyme for a pretty presentation. (optional)
Notes
Use a large casserole dish. Your cottage pie will bubble when cooking and may drip in your oven if your dish is not large enough.
You may want to place a cookie sheet under your casserole dish to catch any drips.
Ensure that you use enough liquid when making your mashed potatoes so that they remain light and fluffy, as they will dry out slightly during the baking process.