DO-AHEAD TIP: Cook pasta shells.
In a large, deep skillet over medium heat, combine first 5 ingredients (ground beef through Greek seasoning); cook for 7 to 10 minutes or until beef is browned; drain.
Stir in tomato sauce then bring mixture to a soft boil. Stir in cooked pasta shells and spinach; cook until shells are heated through and spinach has wilted.
Top with Feta cheese, if desired; serve.