In a large saucepan over medium-high heat, gently simmer carrot, celery, onion and fennel in the stock, covered, until the vegetables are tender, about 15 minutes.
Uncover saucepan. Boil until liquid has reduced slightly, 2 to 3 minutes. Season to taste with salt and pepper. Stir in the butter and serve.
Combining vegetables can make a simple side dish into something much more exciting. Make sure you choose vegetables that will cook in the same amount of time. Try parsnips and carrots, for example, or zucchini and tomatoes. Others can be matched if you parboil the one needing more cooking, such as potato, before combining it with one that cooks faster, such as leeks. Use seasonings that suit both vegetables.