Heat the oven to 375°F. Lightly grease 8 cups of a standard muffin tin.
Place the potatoes and garlic cloves in a saucepan and cover with water. Bring to a boil, then simmer for 15 to 20 minutes, until the potatoes are fork-tender.
While the potatoes cook, combine the ground beef, lentils, mushrooms, onion, egg, oats, 1 tablespoon of the ketchup, thyme, ½ teaspoon of the salt, and the black pepper in a large bowl. Mix gently until evenly combined.
Divide the mixture among the prepared muffin cups, pressing it in lightly. Spoon the remaining ketchup over the tops. Bake for 25 to 30 minutes, until the muffins are cooked through.
Drain the potatoes and garlic well. Return them to the saucepan and mash with the milk, butter, and remaining ¼ teaspoon of salt until smooth.
Let the muffins rest in the tin for 5 minutes, then loosen the edges and lift them out. Serve with the garlic mashed potatoes.