1. In a large pot, heat butter and oil over medium-high heat. Add onion and saute until it starts to soften, about 2 minutes. Add mushrooms, celery, salt, basil, and oregano. Saute until vegetables begin to soften and mushrooms have released their juices, about 5 minutes.
2. Add stock and bring to a boil. Carefully add meatballs and bring back to a boil. Reduce heat and simmer until meatballs are cooked through and vegetables are tender, about 25 minutes.
3. Increase heat to medium-high and bring back to a boil. Stir in orzo and boil until tender, about 8 minutes. Add parsley, lemon juice, 1/4 cup Parmesan, and salt and pepper to taste. Ladle into bowls and pass additional Parmesan at the table.
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