Preheat the oven to 425°F.

To make the meatballs, start by finely mincing the mushrooms using a food processor with the blade attachment.

Combine the finely minced mushrooms with the ground beef, Parmesan, salt, pepper, and Italian seasoning.
Mince the onion and garlic, also using the food processor and add 1 tablespoon of that mixture to the meatball mixture.

Line a cookie sheet with foil and shape 1.5 tablespoon-sized meatballs with your hands or a cookie scoop and place them on the cookie sheet. Bake them in the oven for 20-25 minutes or until brown and release from the foil easily when you try to lift them off with a spatula.

While the meatballs cook in the oven, add the olive oil to a large saucepan over a medium heat and sauté the minced onion and garlic with a pinch of salt and pepper.

When that has softened, after about 4-5 minutes, add the balsamic vinegar, beef broth, and stewed tomatoes. Break the tomatoes up using the spoon so that they are bite-sized pieces.

Put the lid on the pot and let it simmer while the meatballs cook in the oven.

When the meatballs are cooked, transfer them to the pot and stir gently. Let them simmer together for 10 minutes.

Just before serving, add the two cups of fresh baby spinach. If you like, serve with some freshly grated Parmesan cheese.