1. Preheat the oven to 200°C (400°F). Remove the oven rack and cover it with aluminum foil. Take out the butter to soften it a bit.
2. Prepare the ingredients for the croque-monsieur:
Cut the tomatoes in half, remove the seeds, and dice the tomatoes into 1 cm pieces. Set them aside.
3. Cut the slices of ham into 4 pieces and place them on a plate.
4. Cut the mozzarella into small cubes (1 cm in size) and place them in a bowl.
5. Chop the basil leaves and set them aside.
6. Spread butter on one side of the 8 slices of bread.
7. Place 4 slices of bread, buttered side down, on the aluminum foil-covered oven rack. On each slice, place a slice of ham, followed by mozzarella cubes, tomato cubes, and basil. Finish with another slice of ham and cover with a slice of bread, buttered side up. To keep the croque-monsieur closed, secure them with one or two toothpicks.
8. Put the rack in the oven and bake the croque-monsieur for 5 minutes. Flip them over and bake for another 5 minutes. They're ready!