Mash the butter in a bowl with the tarragon, parsley and mustard, as well as some seasoning. Cover and chill until needed. Can be made up to three days ahead.

Trim the long green tops off the carrots and wash under cold running water, scrubbing them with a clean scourer to get rid of any dirt. Halve the carrots lengthways so they're all the same size, and put in a large, microwave-proof bowl with 3 tbsp water. Cover the carrots with a microwaveable plate or lid and heat on full power for 8-10 mins until just tender. Drain, then toss with the herby butter and serve straightaway.