Onions are one of the trigger foods involved in acid reflux. As with chocolate, research shows that meals containing onions weaken the lower esophageal sphincter and lead to symptoms of gastroesophageal reflux disease (GERD), also known as gastro-oesophageal reflux disease (GORD) across the pond.
Effects of antacid formulation on postprandial oesophageal acidity in patients with a history of episodic heartburn. (Link)
The effect of raw onions on acid reflux and reflux symptoms. (Link)