Put the potatoes in a medium pan, cover with water and bring to a boil. Boil until they're very tender, about 15-20 minutes.
Meanwhile make the bottom layer: Put the oil in a large skillet over medium-high heat. When the oil is warm add the onion, spices, salt and pepper and cook until the onion is translucent, stirring occasionally.
Add the mushrooms and saute them for 1 minute. Add the rest of the bottom layer ingredients. Lower the heat to simmer, cover the skillet and cook until the lentils are tender about 20 minutes. Stir occasionally and make sure that the bottom of the skillet isn’t burning. If all the liquid evaporated and the lentils are still chewy, add 2 - 4 tablespoons of water.
Make the mashed potatoes: peel the potatoes and put in a medium bowl. Using a potato masher or potato ricer mash them into the bowl. Add the mashed potatoes ingredients and mix until well combined. Taste and correct seasoning.
Remove the bay leaf and thyme stems. Put the mushroom lentil in a small (8inch / 20cm) round or square ovenproof dish and spread it evenly.
Put the mashed potatoes in a piping bag* fitted with a large flower drop tip. Hold the bag straight but bend your wrist inward. With the tip just touching the surface, hold the top of the bag with one hand and use your other hand to gently squeeze out the mashed potatoes. Stop squeezing before lifting the tip away. Cover the pie with flowers. It’s okay if the flowers aren’t the same size.
Put the pie in the broiler for 10 minutes or until the mashed potatoes get some brown lines or patches.