Heat oven to 325°F/160°C and prepare a water bath.
In separate batches, cook the carrots, green beans and peppers in salted boiling water until tender, 3-5 minutes each. Drain and pat them dry with paper towels. Boil leeks in salted water 8-10 minutes until tender, drain and dry them.
Sauté the mushrooms in the butter until tender and their moisture h