Yes raw onions are very healthy and good for your heart and eating onions often enough could also prevent heart disease which would mean that you would decrease your chances of having a heart attack.
Onions are packed full of nutrients and antioxidants which help to detox your body.
The onions also contain many other compounds that can fight inflammation, decrease triglycerides and reduce cholesterol levels.
When you reduce your cholesterol levels that helps to improve your heart health which reduces your risk of heart disease, heart attacks, strokes etc.
The raw onions also contain anti inflammatory properties that help reduce high blood pressure and protect against blood clots.
When you have lower blood pressure and no blood clots then you prevent heart attacks, heart disease, strokes etc.
High blood pressure is one of the leading causes of heart attacks, heart disease and strokes.