To make cream gravy, melt butter in a medium saucepan. Stir in flour and cook for 1 minute. Gradually whisk in milk. Whisk in chicken base. Bring to a simmer and cook 2-3 minutes. Keep warm.
Preheat oven to 350 degrees. Toast all 8 slices of bread and cut in half diagonally.
Use an oven-safe plate to assemble each sandwich. If you do not have oven-safe plates, use a baking sheet or cake pan. Place triangle slices of bread on the plate, using 4 triangles per serving. You can overlap them or arrange them with the corners towards the center of the plate so that they form a square.
Warm the turkey and ham by microwaving briefly. Place 1/4 of the turkey and ham on top of the bread.
Ladle with cream gravy. Place 2 tomato slices on top of gravy and cover with cheddar and parmesan cheese.
Crisscross 2 slices of bacon on top. Place in oven until cheese is completely melted.
Repeat for remaining sandwiches.
Chicken Base comes in a jar and can be found with the section of the store with bouillon cubes.