Chop the onion, garlic (if desired), and mushrooms
Turn on the Instant Pot on sauté mode and add 2 tablespoons of butter. Once the butter is melted, add the chopped onion, garlic and mushrooms, combine, and let it cook for about 5 minutes, stirring occasionally with a wooden spatula
In the meantime, wash your buttercup or kabocha squash well under running water, rubbing any dirt off the skin with a cloth
Turn off the Instant Pot sauté mode, add 1 cup of bone broth or water, and combine well, scraping down the sides
Place the Instant Pot rack over the mushrooms and onions, and place the whole squash on the rack. Turn on the Instant Pot pressure cook function for 15 minutes (or longer, if your squash is bigger)
Once the pressure cook cycle is over, quick-release the pressure and safely open the Instant Pot lid. Remove the squash with the rack and set aside
Heat your cast iron skillet over medium-high heat and add 1 tablespoon of butter. Once the pan is hot and the butter melted, add the ground meat and the cooked mushrooms and onions (including all the liquid) from the Instant Pot
Season the meat with salt, pepper, and your choice of herbs (I used fresh green onion and parsley and dry sage and rosemary), combine together and let it cook for about 10 minutes, or until most of the liquid evaporates. Stir frequently with a wooden or metal spatula
In the meantime, prepare the squash topping. Once the squash is still hot but cool enough to handle, cut it in half, remove the seeds with a spoon, and remove the outer skin with a knife.
Mash the squash using a fork, add 3 tablespoons of butter, 1 egg yolk, and season with salt to taste. Combine well until the mixture is smooth and creamy
Preheat the oven to 350 degrees F
Remove the cast iron pan from the stove, level the meat with a spatula, and top it with the squash mixture. Spread the squash evenly and create a pattern with a fork, if desired. If wanted, sprinkle the top with grated parmesan cheese
Bake the Sheperd's pie in the preheated oven for about 25 minutes, or until the top is evenly browned. Serve warm