Cooking at low temperature on induction cooktop
You can use a water bath and see if it will stay low enough for the desired time. You can play around with the water amount to achieve more consistency. 

Even with the water bath, you won't get actual 50 degrees for 15 minutes, rather you'll have some slowly rising temperature which will stay in a hopefully acceptable range for the 15 minutes. This is because a cooktop is simply the wrong tool for the job, and especially the induction ones can be worse than the oldtime ones. 

If you really want low temperatures for a sustained time, use an oven (if yours goes that low) or an electronically controlled cooking device.