First of all, sauté the onion, bay leaf, and garlic in a pan with olive oil for about 2 to 5 minutes on medium heat, be careful not to burn the onion. When the onion is soft, add the carrots and mushrooms to the pan and sautée them for more 5 minutes, continually stirring to prevent burning.
Next, add the tomato, tomato puree, and white wine and cook for more 5 minutes, continually stirring until the alcohol has evaporated.
Finally, add the lentils, peas, seasonings, and water, and let it cook for 15 minutes on low heat.
Meanwhile, add the potatoes into a pot, cover with water and cook until potatoes are tender.
Once cooked, drain the water and add the milk, salt, black pepper, nutmeg, and butter. Mash the potatoes until smooth and creamy with the help of a potato masher.
Preheat the oven to 390 degrees F ( 200 C ).
Add the lentil mixture into a baking dish, and then add an even layer of mashed potatoes on top to cover the lentil mixture.
Bake for 20 minutes or until golden.