Make your marinade by mashing the garlic, thyme, rosemary, salt, and pepper using a mortar and pestle. Add olive oil to cover this and mix well.
Using a brush, brush one side of the onion slices and place the oiled side on the grill. Brush the other side. Cook until grill marks are shown and the onion is slightly browned and translucent. Do the same with the zucchini. You may have to cook these in batches, I cooked the onions first, then zucchini, then mushrooms. If you are doing it this way, once the onions are off and the zucchini are on, take the rest of your marinade mixture and using the oil only, mix the rest of the marinade on your mushrooms and let set while the zucchini are grilling. Cook mushrooms and then pull everything off the grill.
Take your garlic bread or french bread and place, cut side down, directly on the grill. If using garlic bread you will probably get flames as the butter and oils drip down onto the coals. Be careful! The bread only needs to be on for a few minutes, until slightly toasted. Watch this carefully.
Remove the bread and construct your open faced sandwich. Bread, zucchini, onions, mushrooms, cheese. Put back on the grill (non-heat side) and cover until cheese melts. Remove and get ready for some goodness.