The recipe

1. Go to the Issues -> Search view and search for "active" issues (it means issues with started timer). 

   Tip
   JQL search term is: timerStarted = "true"

2. You can extend the search according to your liking (e.g. narrow to specific project)
3. Save the search as a filter
4. Go to Dashboard or create new one if you haven't already
5. Add new Gadget of type "Filter results" and select the filter saved in point 2: Adding the Filter Results Gadget

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