4 Tbs. extra-virgin olive oil
2 carrots, cut into 1/4-inch dice
2 celery stalks, cut into 1/4-inch dice
1 onion, cut into 1/4-inch dice
1/2 oz. dried wild mushrooms, soaked in 1 cup hot water,  chopped, soaking liquid reserved
3 garlic cloves, minced
2 1/4 cups fresh bread crumbs
1 Tbs. plus 1/4 cup minced fresh flat-leaf parsley
2 oz. mozzarella cheese, shredded, plus 6 oz., cut into 1/2-inch cubes
Salt and freshly ground pepper, to taste
2 lb. ground beef (80% lean)
2 eggs, lightly beaten
1/2 tsp. minced fresh thyme
Mashed potatoes for serving (see related recipe at left)