Combine the breadcrumbs and milk in a medium bowl and set aside to soak (have some extra breadcrumbs on hand in case you need to adjust the meatball mixture later).
Using the large holes of a box grater, grate the onion and the mushrooms. Heat a large skillet over high heat, add the oil, then add the onions and mushrooms and ½ teaspoon of the salt. Cook, stirring occasionally, until the liquid from the onions and mushrooms has evaporated, about 10 minutes. Set aside to cool completely.
Meanwhile, in a large bowl, combine the ground meats with the remaining ¾ teaspoon salt, grinds of black pepper, eggs, garlic, parsley, mint, and cheese. Squeeze any excess liquid from the breadcrumbs and add them and the cooled mushroom mixture to the bowl. Use your hands to mix well, squeezing the meatball mixture through your fingers so that everything is well distributed. If the mixture seems too wet – ground turkey is often quite moist – add some dry breadcrumbs.
Heat a large skillet over medium heat. Make a small “taster” patty and fry it in a little oil on both sides until cooked through. Taste it and adjust for salt and texture – if the mixture seems too wet, mix in some more breadcrumbs; too dry, a little splash of milk. Use your hands to form the mixture into balls. Refrigerate if not cooking soon.
Reheat the skillet to medium and add the meatballs, starting at one side and working to the other. As they brown, turn them in the order they went in. Turn and brown 3 or 4 times, adjusting the heat as needed so that they cook through without burning. Break one open to check for doneness.
If you’re planning to eat the meatballs with tomato sauce and pasta, add your sauce, or just some chopped tomatoes, to the skillet and let it all simmer while you cook the pasta. Add a splash of water if needed to keep it saucy.
• Mushroom meatballs are good in a hot sandwich with tomato sauce and melted cheese.
• Make them smaller (walnut-size), sprinkle with grated cheese or chopped herbs, and serve as an appetizer.
• Meatballs freeze well too, cooked or raw.