1. In 10-inch skillet over high heat, cook ground beef and onion until all pan juices evaporate and meat is well browned. Spoon off any fat. Pour mixture into large bowl. Stir in ricotta, spinach, mozzarella, and salt.
2. On waxed paper, brush 1 sheet of phyllo with melted butter or margarine; place in 9-inch pie plate, with edges overhanging. Brush second sheet with melted butter; place over first sheet with corners angled slightly away from corners of first sheet. Continue layering, brushing each sheet with butter and placing sheets so that corners are always angled away from corners of sheet directly underneath.
3. Spoon beef mixture onto phyllo in pie plate. One by one, fold overhanging edges of phyllo over beef mixture until top of pie is covered. Brush top with remaining melted butter. Bake in 350°F. oven 1 hour or until pie is golden.