Preheat the oven to 375°F.
In a large bowl, combine the ground beef, onion, garlic, eggs, cheese, Italian seasoning, salt, and pepper. Add the mushrooms to the meat mixture. Slowly stir in 1/2 cup marinara, and combine well. Add another 1/2 cup of marinara and combine again. The mixture should be very moist but still able to hold its shape when rolled into meatballs.
Form the mixture into 2-inch meatballs. Arrange the meatballs in a single layer on a rimmed baking sheet. Bake for 35 minutes, turning occasionally, until evenly browned.
Serve with marinara. Store the meatballs in an airtight container in the refrigerator for up to 4 days or in the freezer for up to 2 months. To reheat, place in a preheated 350°F oven for about 3 minutes, until heated through.