Can onions cause heart palpitations?

No: Haven't heard of onion palpitations. Green onions and yellow onions aren't spicy enough to create the excitement that might lead to sensations of palpitations. Eating cooked onions should be fine, just like eating other cooked vegetables. Theoretically, getting hit in the heart by a big yellow onion flying at high speed can cause a problem, similar getting hit by a baseball, so don't throw onions.