Preheat conventional oven to 350°F.
In a medium sized bowl, lightly mash the avocados. Stir in onions, parsley and relish. Cover and refrigerate 1 hour to blend flavors.
To assemble sandwiches, place 2 slices of Cheddar cheese on one half of flatbread, spread 2 tablespoons of avocado relish on top of the cheese. Place 2 to 3 slices of tomato on top. Place Swiss cheese slice on top of tomatoes. Fold flatbread in half. Lightly spray with cooking spray.
Arrange flatbread sandwiches on full sheet pan; placing another sheet pan on top to slightly flatten sandwiches and help in melting cheese. Bake for 10 to 12 minutes until cheese is melted.
Cut into wedges.
CCP: Heat to 165°F for at least 15 seconds
CCP: Maintain hot food at 135°F or above