Preheat the oven to 375°F and line a baking sheet with parchment paper.
Heat a skillet over medium heat and cook the mushrooms, onion, and garlic until the mushrooms release their liquid and the onion softens, about 6 minutes.
Transfer the cooked vegetables to a large bowl and add the lentils, breadcrumbs, egg, thyme, salt, and pepper. Stir until the mixture holds together.
Divide the mixture into four equal portions and shape each into a small loaf on the prepared baking sheet.
Place the potatoes in a pot of salted water and boil until tender, about 15 minutes. Drain and return them to the pot.
Add the butter and milk to the potatoes and mash until smooth. Season with a pinch of salt if needed.
Spoon the mashed potatoes evenly over the top of each mini meatloaf.
Bake for 20 minutes until the tops are lightly golden. Sprinkle with parsley before serving.