Preheat the oven to 400 F.
In a food processor with the blade attachment, add the mushrooms and pulse until they are smaller than a peppercorn.
In a medium bowl, mix the ground mushrooms with the ground beef, rice, spices, salt, and Worcestershire Sauce with your clean hands. Using your hands is really the best way to make sure it is all mixed thoroughly.
Set the meat and mushroom mixture aside and mince the onion and garlic in the food processor (no need to clean it out) or chop the onion and garlic by hand if preferred.
Heat the olive oil in a frying pan and sauté the minced onion and garlic over medium heat for 3-4 minutes.
Mix the sautéed onion and garlic with the crushed tomatoes, broth, dried herbs, and salt & pepper in a large glass measuring cup or a bowl. Stir well and then transfer half the liquid to a large rectangular casserole dish.
Scoop out and shape slightly heaped tablespoon-size meatballs and place them in the dish on top of the sauce. Do not make them too large or the rice won’t cook properly.
Once all the meatballs are shaped and in the dish, pour the remaining sauce over top of them. Don’t worry if they are not completely covered in the sauce.
Use either the lid to the casserole dish or snugly fit a large piece of foil over the dish.
Bake in the oven for 40-45 minutes at 400 F. You can take the foil (or lid) off when there is about 10-15 minutes left if you want the sauce to thicken slightly.