Pulse the mushrooms in food processor until roughly chopped, similar to the size of pine nuts (Alternatively, you can hand chop the mushrooms if you don’t have a food processor).
Heat a drizzle of avocado oil in a large pan over medium-high heat. Add mushrooms and a decent pinch of Kosher salt and stir. Spread out into a thin layer and cook for 3 minutes without touching. Stir mushrooms and spread out into a thin layer again. Cook for an additional 3 minutes. Then cook for another 1-2 minutes, stirring occasionally, until lightly browned and tender. Transfer to a plate, season with Kosher salt, if needed, and let cool completely. Wipe out pan to use for meatballs.
Once the mushrooms have cooled, form meatballs. In a large bowl, add the grated onion, grated garlic, Parmesan, breadcrumbs, oregano, parsley, and water. Mix until evenly combined. Add the whole egg, egg yolk, ground turkey, a generous amount of Kosher salt (I used 2 teaspoons for 16 meatballs), and freshly ground black pepper. Mix well with your hands for 30 seconds - 1 minute.
TIP: I recommend taking a tiny portion of the meatball mixture and cooking it in a pan with oil until cooked through, to taste and ensure the mixture is seasoned to your liking.
Scoop into meatballs a little larger than golf balls (I use a 2 tablespoon-sized cookie scoop and overfill them a bit), about 1 ½ inches in diameter. It helps to rub your hands with oil before rolling, the mixture will be sticky.
Coat the bottom of a large pan with a thin layer of avocado oil and heat over medium-high heat. Add the meatballs, spacing them evenly, and cook for 6 minutes. Flip them every two minutes so that they’re nicely browned on multiple sides.
Add tomato sauce to the pan with the meatballs, lower the heat to low, and bring to a simmer. Cook for 18-22 minutes, until meatballs are cooked through. Add in a little bit of water if sauce thickens too quickly. Taste and season with Kosher salt and freshly ground black pepper, if needed.
Serve hot sprinkled with fresh herbs and Parmesan cheese.