With lentils and cremini mushrooms as well as the traditional carrots and peas, this vegetarian take on the traditional British potato-topped meat pie is hearty and delicious. We use French green lentils because they hold their shape in cooking.
Heat oil in a medium pot over medium. Add onion, carrots, and celery and cook, stirring, until tender, 6 to 8 minutes. Add mushrooms and cook, stirring, until tender, 8 to 10 minutes. Add tomato paste and cook, stirring, 1 minute. Add Worcestershire, bay leaf, 4 cups water, and lentils; season with salt and pepper. Bring to a boil, then reduce to a simmer and cook until lentils are tender, 30 to 35 minutes. Stir in peas; remove from heat and season with salt and pepper.
Preheat oven to 400 degrees. Place potatoes and garlic in a medium saucepan; cover with 2 inches of cold water. Season generously with salt; bring to a boil, then reduce to a simmer and cook until a knife pierces potatoes easily, 10 to 12 minutes. Drain; return potatoes and garlic to pot. Add butter and milk; mash with a fork or potato masher until smooth. Season with salt and pepper.
Transfer lentil mixture to a 2-quart baking dish. Dollop with mashed potatoes; spread evenly to edges. Transfer to a baking sheet; bake until golden and bubbling around edges, about 15 minutes. Let stand 20 minutes; serve.