Preheat oven to 325F. To prepare the guavas for stuffing, cut a thin slice from the top of the guava. With a melon ball scoop or small spoon, remove the seeds and set the guavas aside. Heat the oil in a skillet. Add mushrooms, onion, green olives, herbs, salt and pepper; saute until vegetables are soft. Remove the mixture from the flame; cool. Stuff the guavas, filling them to the top. Place stuffed guavas in a baking dish just large enough to hold them and sprinkle sesame seeds over the guava tops. Bake for 1 hour, or until the fruit is tender. Serve hot.