Bubble under
Definition 1 of 2
Pro Tip 1/3Light Boiling
Imagine tiny bubbles constantly rising, but slower than a full boil. 
Keep the stew bubbling under while you chop the vegetables.
Pro Tip 2/3Continuous Process
Think of 'bubble under' as a steady, ongoing activity, not starting and stopping. 
I left the broth bubbling under for hours to enhance its flavors.
Pro Tip 3/3Subtle Action
'Bubble under' suggests a gentle action, less intense than 'boil'. 
The sauce should bubble under, not erupt like a volcano!