Peel and dice the potatoes. Place in a large saucepan with enough water to cover. Bring to a simmer, then cover and simmer until tender, about 20 minutes. Drain and transfer to a small mixing bowl.
Stir the vegan butter into the potatoes until melted, then add the rice milk and mash until fluffy. Season with salt, cover, and set aside until needed.
Preheat the oven to 400º F.
While the potatoes are cooking, heat the oil in a medium skillet. Add the onion and sauté over medium heat until translucent. Add the garlic and mushrooms and continue to sauté until the onion is golden.
Add the lentils and their liquid and bring to a gentle simmer. Stir in the optional wine, soy sauce, seasoning blend, thyme, and pepper. Cook gently for 5 minutes. Combine the cornstarch with just enough water to dissolve in a small container. Stir into the lentil mixture.
Add the spinach, a little at a time, cooking just until it’s all wilted down. Remove from the heat; taste to adjust seasonings to your liking.
Lightly oil a 2-quart (preferably round) casserole dish, or two deep-dish pie plates.
Scatter the breadcrumbs evenly over the bottom. Pour in the lentil mixture, then spread the potatoes evenly over the top. If using two pie plates, divide each mixture evenly between them.
Bake for 30 to 35 minutes, or until the potatoes begin to turn golden and slightly crusty. Let stand for 5 to 10 minutes, then cut into wedges to serve.