Preheat oven to 425 degrees F.
Combine zucchini and yellow squash pieces with onions and mushrooms in a mixing bowl. Add oil to the bowl, 2 turns of the bowl in a slow stream. Toss veggies until they are coated lightly and evenly with the oil.
Combine poultry seasoning with salt and pepper and sprinkle evenly over the vegetables. Toss them again to combine and spread them out on to a baking sheet. Place veggies in a hot oven and roast 20 to 25 minutes until tender. Toss and turn the veggies with kitchen tongs half way through cooking time.