In a small bowl, stir together mayonnaise and basil.
Spread each slice of bread with an equal amount of the basil mayonnaise.
Divide the tomato slices evenly between two pieces of bread. Season tomatoes with salt and pepper. Place the other two slices of bread (mayonnaise-side down), on top of the tomatoes to form two sandwiches. Slice in half and serve.
These sandwiches are best served immediately. While you can certainly pack them in a picnic basket for the pool (I've done it many times!), the bread tends to get soggy from the tomatoes after too long.
This recipe yields two sandwiches; however, you can easily scale the recipe up to make as many sandwiches as you need.
Don't forget to season the tomatoes with kosher salt and freshly-ground black pepper. This extra touch adds such great flavor to the sandwich!
Feel free to add extra basil leaves to the sandwich if you like, or adjust the amount of mayonnaise to suit your tastebuds.