Preheat oven to 400º F. Start cooking the potatoes: Wash and add to medium saucepan filled with cold water. (See Notes for Russet potatoes.) Bring to boil over high heat on stove. Let boil until potatoes are soft enough to be easily pierced with a fork, about 12-20 minutes. Continue with next steps while potatoes cook.
Cook vegetables: Melt 2 tablespoons butter or vegan butter in large skillet over medium heat. Add onion and cook until translucent. Add garlic and mushrooms. Sauté, stirring constantly, until garlic is fragrant, mushrooms are soft, and onion is golden.
Add lentils and their liquid to skillet. Turn down heat so liquid gently simmers. Stir in wine, soy sauce, rosemary, and dried spices. Cook 5 minutes. Add cornstarch or arrowroot to small bowl and mix with just enough water to dissolve. Pour into skillet and stir. If using spinach, add a little at a time and cook until just wilted. Remove skillet from heat and remove rosemary sprig and discard.
Make mashed potatoes: When done boiling, remove from hot water with slotted spoon and place in ice water bath. Remove skins after minute or so. Cut the potatoes into quarters.
Microwave 1/4 cup butter or vegan butter and milk or plant-based half and half in microwave-safe bowl or glass until butter melts and milk is hot. Pour into potatoes and use masher or back of spoon to mash to desired consistency. Add more milk or half in half if potatoes are lumpy. Season with salt to taste.
Prepare pie: Lightly oil or grease 2-quart casserole dish (preferably round), two deep-dish pie pans, or cast iron skillet. Casserole dish or pie pans: Scatter breadcrumbs over greased bottom. Pour in the lentil mixture. Cast iron skillet: Pour in lentil mixture, then sprinkle breadcrumbs over the lentils. After lentils and breadcrumbs have been added, spread mashed potatoes evenly on top.
Baked 35 minutes or until top becomes slightly crusty and golden. If desired, brush melted butter or vegan butter on top of potatoes 20 minutes into baking time. When done, remove from oven and let stand 5-10 minutes. Cut into wedges and serve.