Cut off ends of zucchini; halve zucchini lengthwise.
In covered skillet cook zucchini in a small amount of boiling salted water for 5 to 10 minutes or till crisp-tender.
Scoop out pulp, leaving 1/4 inch shell.
Chop enough zucchini pulp to make about 3/4 cup; set aside.
In small skillet cook onion in butter or margarine till tender.
Combine provolone cheese, mushrooms, flour, parmesan or romano cheese, sour cream, salt, basil, and the reserved zucchini pulp.
Add cooked onion; mix lightly.
Fill zucchini shells with cheese mixture; arrange in 10x6x2-inch baking dish.
Bake, uncovered, in 350° oven for 20 to 25 minutes or till heated through.