Layer onions in an even layer in the Instant Pot®. Scatter the garlic on top of onions. Add red potatoes in an even layer, then do the same for the corn and sausage. Add the shrimp and sprinkle with Old Bay Seasoning. Pour in broth.
Squeeze lemon wedges into the Instant Pot® and place squeezed lemon wedges into the Instant Pot®. Lock lid.
Press the Manual button and adjust time to 5 minutes.
When timer beeps, quick-release the pressure until float valve drops and then unlock lid. Transfer ingredients to a serving platter and garnish with parsley.