Prepare Meatballs: In large bowl, combine ground beef, egg, breadcrumbs, onions, water, cilantro, salt and pepper; mix well. Shape 2 tablespoons of the meat mixture into a ball; with thumb make large hole in center of meatball. Place about 1 tablespoon of shredded Queso Blanco in center of meat. Close meatball to enclose cheese.
Prepare Sauce: In food processor or blender combine spinach, Cotija cheese, walnuts and garlic; pulse until spinach is chopped. With the motor running, slowly drizzle in olive oil until well blended. Add salt and pepper to taste.
In 12-inch skillet over medium-high heat, in hot vegetable oil, cook meatballs until well browned on all sides and cooked through, stirring frequently, about 10 minutes.
To serve, spoon sauce on platter. Top with meatballs.