Slice the zucchini lengthwise using a vegetable peeler or mandolin. Finely slice the chicken breasts. Overlap slices of zucchini on a bamboo sushi mat. Top with slices of chicken. Season with salt and pepper. Spread the ricotta on the chicken. Arrange a row of basil leaves and a row of chopped tomatoes on top. Roll up and put in the steaming basket. Fill the casserole with 3 quarts water. Bring to a boil and place the covered steaming basket on top. Steam for 15 minutes. Let the roll cool a bit before slicing.