Combine the garlic, bay leaf, carrot, celery, onion and oil in a medium skillet and cook, covered, over low heat, stirring occasionally, until the vegetables are softened, about 10 minutes. Push the vegetables to the perimeter of the pan and add the lamb. Increase the heat to high and cook, breaking the meat up with a wooden spoon, until browned all over. Sprinkle with the mustard, 1 teaspoon of the salt and some pepper. Stir until fragrant, 1 minute, then add the tomato paste, fennel seeds and tomato puree and stir again. Cook for 1 minute more. Turn off the heat and stir in the mint and sage. Remove the bay leaf.
Meanwhile, put the potatoes in a medium saucepan and cover by 2 inches with salted water. Cover and bring to a boil. Cook until a knife releases easily from the potatoes when pierced in the center, 12 to 15 minutes. Reserve 1 cup of the cooking water and drain the potatoes. In the same pot, combine the milk, butter, and 1/2 cup of the cooking water and stir together over high heat. Bring to a boil. Mash the potatoes with a masher, then slowly drizzle the hot liquid over the potatoes and stir until light and fluffy. Taste and season with 1/4 teaspoon salt and pepper to taste.
Spoon the lamb mixture into the bottom of an 8-inch deep-dish pie plate, spreading it evenly over the bottom. Spoon the mashed potatoes over the top, covering completely. Use the back of a spoon or an offset spatula to create a swirled pattern in the potatoes. Bake until the topping turns golden in spots, about 30 minutes. Serve warm.