can I cook potato halves, onion slices and sliced carrots together in one pot with just some butter (with no water, but lid on, so the vegetables would steam in their own vapour)? If so, I assume they will need different cooking times. Approx. how long would you cook each of the different vegetables? My guess would be 30 minutes for the potatoes, and 10 minutes for onion and carrots. Is this correct?