Spread butter on one side of each of the slices of low carb bread. We love the Sola Brand low carb bread or you can make your own low carb bread recipe: https://lowcarbinspirations.com/deidres-low-carb-bread-recipe-made-keto/
Slice the tomatoes and Mozzarella cheese. Set aside.
Use a small frying pan and heat it over low to medium heat.
Place the butter side down on the preheated frying pan.
Spread a layer of Basil Pesto over the first slice of bread.
Add two slices of the Mozzarella cheese.
Add 2 to 4 slices of Roma tomato (depending on the size, just make sure it's enough to spread over the whole slice of bread)
Add about 2 leaves of fresh basil.
Now add the other slice of bread on top of all of those layers but make sure the butter side is up so that when you flip the sandwich it will be butter side down on the pan. You can add the optional mayonnaise spread to the opposite side of the butter bread if you are using it.
Fry the grilled cheese sandwich long enough for the bread to be a golden brown color on one side and then use a spatula to flip it.
Cook it on the other side until golden brown too.
Cut it in half and serve it warm or cold.
Notes
Note: The low carb bread is not calculated in this because there are many different brands of low carb bread with different calorie and carb counts.