This is a great fall comfort food. It comes from David Tanis in NYTimes Cooking and can be adapted for whatever you may have in the fridge at the time. Served on its own, it is more than filling and very delicious. The thickened white sauce is an important component to the texture and taste and adds a great deal to everyone’s enjoyment.
For this you’ll need: x2 large russet potatoes (peeled and cut into large chunks), x2 leeks (chopped), x1 onion (chopped), x2 garlic cloves (crushed and chopped), 0.5-0.75 lb of mushrooms (cut into slices, we used shiitake and white, but the more varieties the merrier), 2-3 Tbsp fresh thyme, 1 Tbsp fresh parsley, 2 Tbsp white miso (optional, but adds flavor), 2 Tbsp cornstarch (dissolved in 2 Tbsp water), 2 cups whole milk, 4 Tbsp butter, 1/2-1 cup heavy cream, olive oil, bread crumbs, salt and pepper.
Preheat the oven to 350.
In a large skillet add about 2 Tbsp olive oil and 1 Tbsp butter and melt butter over medium heat. Add chopped onion with some salt and pepper and cook, while stirring, until just starting to change color – about 3-4 mins.
Add chopped leeks and a little more salt and pepper and continue to cook over medium heat until all of begun to soften but the leeks retain their color. About another 3-4 mins. Remove skillet from heat and set aside.
In a separate skillet put another 2 Tbsp olive oil plus 1 Tbsp butter over medium heat until the butter melts. Add the chopped mushrooms (you may need to cook these in batches if the skillet isn’t large enough) and cook until browned – about 10 mins – stirring and tossing occasionally. Once browned, turn off the heat and add the chopped garlic and fresh thyme and parsley. Stir well to combine everything.
Meanwhile, start a large pot of water on to boil. Once boiling add the peeled and chunked potatoes, bring the water back to a boil, lower the heat slightly and allow them to cook gently until softened – about 15 mins.
Place the browned mushrooms into the same skillet with the leeks and onions (or vice versa depending on which is you larger skillet), and combine well over medium heat until warmed through.
Add milk, miso (if using) and cornstarch dissolved in water. Stir well and consistently until the whole thing begins to thicken, about 3-4 mins. Remove from heat and set aside at room temperature.
Once the potatoes are soft, strain off all of the water and place them back into the pot you boiled them in. Add 2 Tbsp butter and the cream along with some salt and pepper and mash them with a potato masher or large fork until smooth. Adjust the cream and seasoning to your tastes.
Place the leek/onion/mushroom mixture into a 9×13 inch baking dish and evenly distribute. Gently spread the mashed potatoes over the surface and smooth with the back of a wooden spoon. Add some more fresh thyme and bread crumbs to the surface along with a couple dollops of butter and place the baking dish into the oven.
Bake until brown and bubbly – about 45 minutes.