Rich, warming deliciousness is the order of the day with this vegetarian mushroom shepherd's pie. (We know, we know… not possible to have a vegetarian mushroom shepherd's pie. Work with us here.) A filling made of mushroom, carrot, onion, and celery is topped with creamy mashed potatoes and cheese, then baked until piping hot. The results may not technically belong to any shepherd, but you'll be proud they belong to you.
Peel and cut potato into 1" dice.
Halve and peel onion. Cut halves into ¼" dice.
Cut mushrooms into ¼" slices.
Trim ends off celery and cut into ½" dice.
Make the Mashed Potato
Bring a medium pot with potato covered by lightly salted water to a boil. Cook until tender, 18-20 minutes.
Drain in a colander and return to pot. Add sour cream, butter, half the cheese (reserve remaining for topping), and ¼ tsp. salt. Mash until smooth.
While potato boils, cook filling.
Start the Filling
Place a large non-stick pan over medium-high heat and add 4 tsp. olive oil. Add onion, mushrooms, celery, ½ tsp. salt, and a pinch of pepper to hot pan. Stir occasionally until vegetables are softened, 6-8 minutes.
Add matchstick carrots and tomato paste. Stir until combined, 1 minute.
Finish the Filling
Sprinkle flour over vegetables in pan and stir until thoroughly incorporated, 45-90 seconds.
Add 1½ cup water and stir occasionally until slightly thickened, 3-5 minutes.
Remove from burner.
Bake the Pie
Transfer filling to prepared casserole dish and place on prepared baking sheet to catch any drips. For best results, use either a 4-cup casserole dish or 8" cast iron skillet.
Top with mashed potato and remaining cheese. Bake until golden brown, 10-12 minutes.
Serve family-style. Bon appétit!