If you are not using a cast iron skillet to make this dish, transfer the lentil mixture to an oven safe dish before topping with the potatoes. Use a medium sized dish, the exact measurements are not important for baking this dish. You don't want to use a dish that is too small and have the ingredients overflowing in the oven. Alternatively, you don't want to use a dish that's too large and have the lentils and potatoes spread out too thin. Use your best judgement here.
Keyword comfort food, Lentils, Mushrooms, potatoes, shepherd's pie