Firstly turn your oven on 180 degrees. Next chop your zucchini in to half and then slice in 1cm pieces, now chop your kohlrabi in similar sizes and break your florets into small florets. Then in a bowl place your tbsp of olive oil and add some salt and pepper and toss your kohlrabi and place in a roasting dish and place in oven for 30 minutes. Then in the same bowl toss your cauliflower and zucchini in the oil. After 30 minutes check on your kohlrabi and toss. Wait another 10 minutes then add your cauliflower and zucchini and continue to roast for 20 minutes.