After you start a recording, you can Pause, Resume, or "STOP & RESTART" a recording, by pressing the Side button and the Crown together.

Be sure to have it activated "Side + Crown" on the Settings page.

If it doesn't work, be sure to have it activated in System Settings/Workout, on your Watch.

Double Click: to avoid not intentionally pressing the buttons, set Double Click, to click twice consecutively to activate the action.