Marinate the mushrooms long enough to coat, and remove with a slotted spoon. Do the same with the squash, keep separate.

Assemble six slices of bread on a large baking sheet. Place mushroom slices on each slice of bread, if you cover the whole slice of bread there is less chance of the crust ends burning. But if you let a little crust poke through, it might get nice and crisp.
Then layer the summer squash over the mushrooms.

Place in 350° F. oven and roast for 5 minutes or so. Remove from oven and lay on the tomato slices. Stick this in broiler for a few minutes. Serve hot.
(Note if you pour any marinade over the bread, you run the risk of the sandwiches falling apart).
Proportions in marinade are up to you.