Peel and finely dice the onion. Wash. core and dice the tomatoes. Place the spinach in a mesh strainer. Press or squeeze to remove the liquid. Shred or cut the cheese into smaller pieces to make melting easier. Using a double boiler, heat the milk. Add the cheese and stir constantly as the cheese melts. When the cheese is completely melted, remove the mixture from the heat, Fold in the onions, tomatoes and the spinach. Stir to combine. Serve warm with pita chips, crackers or tortilla chips. Serve warm. If you are bringing this to a party, heat in a 250 degree oven for 10-15 minutes before serving.