Peel tomatoes, slice 3/4 inch slices and set aside to drain.
Place tomatoes in bottom of pie shell (1 layer), top with chopped onion then salt and pepper to taste - sprinkle with basil.
Mix cheese and mayonnaise and spread over the onions to make a top crust from edge to edge of piecrust. Bake at 350 for 30-35 minutes. Top will be bubbly when pie is ready. Wait 15 minutes before serving.