Peel onions and cut a 1/4-in. slice from the top and bottom. Carefully cut and remove the center of each onion, leaving a 1/2-in. shell; chop removed onion.
In a large skillet, saute the zucchini, garlic and chopped onions in oil until tender and juices are reduced. Stir in the basil, thyme, salt and pepper. Remove from the heat. Stir in the bread crumbs, bacon and Parmesan cheese. Fill onion shells with zucchini mixture.
Place in a greased 3- or 4-qt. slow cooker. Add broth to the slow cooker. Cover and cook on low for 4-5 hours or until onions are tender.