Zucchini Tomato Gratin

I recently bought the book Chez Panisse Vegetables, recipes from Alice Waters' restaurant in Berkley (http://chezpanisse.com/). It's stellar. I've tried several of the recipes so far, the most frequent being the zucchini and tomato gratin. I love it, it's so good. I think the secret is the sauteed onions. And herbs. And tomatoes... you get the idea, the whole thing is just so good! Although technically I'm not sure it's a gratin, it's more of a caserole or a bake. But I'm in no position to argue semantics with Alice Waters!

Today I made it heavy on the zucchini, since I had a whole one to use up but didn't want to make a huge batch to compensate. Usually it's about half and half zucchini and tomato. Here's the recipe as I made it today.

1/2 a large onion
1 tsp butter
approx. 10 sage leaves, chopped
1 medium zucchini
1 medium tomato
balsamic vinegar
olive oil
10 white mushrooms (or are they called brown mushrooms?)

Saute onions in the butter on a low temperature for about 15 minutes, until soft. Add the herbs, continue to heat for about a minute, then pour into a small baking dish, about 6 inches in diameter.

Cut zucchini and tomato into thin (1/4 inch) slices. Layer the tomatoes on top of the onions, then drizzle a bit of balsamic vinegar over them and salt and pepper. Then layer the zucchini over that. This might be two layers of zucchini. On top of each layer, drizzle some olive oil, then salt and pepper.

Chop the mushrooms very small (I used a small food chopper but a knife would be fine) and spread over the gratin. Cook at 350 degrees for 30-45 minutes, until the juices are bubbling.

Zone-wise, this is about 2.5 carb blocks, with higher fat and no protein.