No, you can hold onto it. Here.

[He steps up behind her and wraps his arms around her frame so that his hands guide hers.]

Alright, take this tomato like so...

[He has her pick up the tomato in one hand, then guides her other hand with the knife. He angles it towards the top of the tomato and carefully, but skillfully, guides her into cutting a little cone at the top, popping the stem out with minimal loss to the vegetable itself.]

See? If you do it like that, you don't lose any vegetable and you're sure to get the whole stem.

[He looks down at her and smiles. Does he even realize how this looks? He might in a moment.]