A soft vegetable like zucchini becomes soggy because it has a lot of water. When you allow it to cook for a long time over low heat, the water breaks out and makes everything soggy. Roasting veggies should be done over high heat and quickly. This way, the outsides get crisp while any leaked water evaporates rather than residing in the container. For roasting zucchini, do it at 400 °F, and with coin-sized pieces, it will take just 5 minutes. That is enough to cook the zucchini without spilling the water. Also, broiling for a couple of minutes will roast it enough to get the right color.