Add beer, quartered onion, and crushed garlic to Instant Pot.
Cut rib slab in half if needed to fit. Place ribs in pot.
Cook on meat setting (or high pressure) for 15 minutes after pressure builds. For fall-off-the-bone texture, cook 20-25 minutes.
Let pot sit 5 minutes, then release pressure.
Carefully remove ribs to foil-lined baking sheet.
Brush glaze generously over entire surface of ribs.
Broil on high heat until glaze caramelizes, about 5 minutes. Watch closely to prevent burning.
Cut into individual ribs and serve immediately.