Preheat oven to 375F.
Pour broth in the insert of your Instant Pot.
Place steamer basket inside the Instant Pot.
Transfer prepared cauliflower to the basket inside the Instant Pot.
Close the lid and set the valve to sealing.
Set on manual setting and cook on high pressure for 3 minutes.
Use the quick release function to release the pressure.
Drain out all of the liquid and transfer the cauliflower to a food processor or blender.
Add garlic, butter, sour cream, dry mustard, chili powder, salt, and pepper to the blender; process until blended and creamy. I recommend using the pulse function because you have more control over how creamy or chunky you want your mashed cauliflower.
Transfer the cauliflower mixture to a baking dish.
Top with cheeses and prepared bacon; bake for 5 minutes, or until cheese is melted.
Sprinkle green onions over the top.