Prepare the potato topping: Place potatoes in a large saucepot and cover with 1 inch of water. Add 2 Tbsp salt and bring to a boil. Cook until fork-tender, about 20 minutes. Mash potatoes with sea salt, butter, milk, pepper, parsley and egg.

Prepare the filling: Place mushrooms, onion and mixed veggies in a 4-quart Dutch oven or 9 × 13-inch spray-coated ceramic baking dish. Add olive oil and sea salt, and mix until thoroughly coated. Bake at 425 F for 15 minutes or until mushrooms reduce in size by about half.

Whisk together broth, flour and Worcestershire sauce. Pour sauce over vegetables and bake in center of oven for about 3 minutes, until sauce has thickened.

Remove baking dish from oven. Spoon potato mixture over all and gently spread it out evenly. Bake 20 minutes to set potatoes, then broil 1 to 2 minutes to brown the top.