In a large oven proof casserole, drizzle with a bit of olive oil and add the cubed zucchini.
Slice the onions thinly. Heat the remaining oil and butter in a pan and slowly cook the onions to caramelize them. Once the onions are done scatter them on top of the zucchini.
For the topping combine all the ingredients and distribute evenly over the zucchini and onions.
Season with salt,pepper, garlic powder to taste.
Add more olive oil if it seems too dry.
Bake at 370 degrees for 40 minutes