The force that pushes the inside section of the onion out through one of the polar openings is pent-up steam. To avoid this, give the steam one or more extra escape routes by piercing the onion with a thin skewer before you add it to the water or stew. Another popular countermeasure is to make a 1/4 inch deep, X-shaped incision into the root end. Whatever technique you employ, simmer the onion, because boiling increases the chances of onion-pop by bouncing the bulb around the pot.