Pistachio pesto: add basil, pistachios, garlic cloves, and salt to a food processor or blender. Pulse to combine.
When things are chopped well, start slowly adding your olive oil while the processor/blender is running on low. This starts to form an emulsion and gives you the consistency you would want.
When the pesto is smooth and has a thickened consistency, remove from the processor/blender to a bowl add parmesan cheese, and stir.
Taste the fully combined pesto and add any additional salt and pepper that you think it might need. Store in the fridge for a few days.
Roasted Roma tomato slices: Slice the Roma tomatoes and place them on a sheet pan with at least a half inch of room between each one. Paint or spoon a little olive oil on top of each tomato and then sprinkle a tiny pinch of salt and ground black pepper on top of each tomato. Bake them at 400 degrees F (205 C) for 15 to 20 minutes or until the tomatoes are soft but not falling apart. Remove them to a plate until it's time to make the sandwich.
Toast the bread slices: place a griddle or large pan over medium heat to preheat for 5 minutes. After 5 minutes have passed, add the bread slices covered with 1 teaspoon of butter per side. Cook, flipping occasionally until the bread is golden brown and toasted. Remove the bread to a cooling rack, and keep the griddle or pan on the heat.
Turkey and cheddar: wipe out the griddle or large pan and add several slices of turkey. Cook until the turkey is fully warmed through and starting to brown a little bit.
Add the slices of cheese on top of the hot turkey and cook for another 2 to 3 minutes until the cheese starts to melt.
Sandwich assembly: spread a generous amount of pesto on one slice of bread and top that with the turkey and melted cheddar.
Add 3 to 5 slices of baked Roma tomatoes on top of the cheese. Add mayonnaise to the second slice of bread (if using) and add that piece on top of the tomato slices to complete the sandwich.