Bring a heavily salted pot of water to a boil. Cut the potatoes into 1/2" cubes and boil until just tender. Drain into a colander and set aside to steam.
Put enough duck fat into a heavy skillet to coat the bottom. Med-High heat. Add the mushrooms, cook them until golden, turn each over as they brown and make sure both sides are crisp. This seems like a pain but it is worth is for crispy mushrooms! When they are done remove them to a paper towel covered plate.
Add 2 or 3 big scoops of duck fat to the pan. Add the potatoes, and a good pinch of flaky salt. Allow them to sit and sizzle until they begin to brown. Then start pushing them around - you will likely need to add more duck fat as you go - I added 3-4 heaping spoonfuls. Move them around every few minutes so more and more crispy bits develop - I use a wooden spatula and scrape up from the bottom of the pan. After a few minutes, add in the shallots. Once those have softened (and your potatoes are still browning) add in the herbs, and taste for salt and pepper.
Once everything is crispy and brown and nice, add the mushrooms back in and get them hot again. You should now have a big pan of crispy salty duck fatty nirvana.
I work in databases by day, but creativity is my outlet. Food - imagining it, making it, sharing it. And art, I come from a family of artists and have been collaging in my garage studio. You can see my work on Etsy in my shop AbbiesGarage https://www.etsy.com/shop/AbbiesGarage?ref=search_shop_redirect