Cut zucchini lengthwise in halves. Scoop out pulp, leaving 1/4 inch wall; chop pulp and reserve. Place zucchini shells cut sides down in dish. Cover loosely and microwave on high (100%) until crisp-tender (about 5-6 minutes.) Cover and microwave reserved pulp, the onions, and butter in a 1 1/2 quart casserole or bowl on high until tender (6-7 minutes.) Stir in cheese, tomato, salt and pepper.
Turn zucchini shell cut sides up; spoon mixture into shells. Cover loosely and microwave on high until hot (2-3 minutes.)
Makes 8 Servings. 3 net grams of carbohydrate per serving.