Grill enough chopped onion with middle or weak fire in a pan patiently. Onions get sweet when you grill them but bitter if you burn them.
When onions get enough cold, mix them with everything else in a bowl. Form them by hands by doing something like catching balls to exhaust air in them.
Grill them in a pan 'til it gets brown at the surface each side. When you get them upside down after one side got brown, you just need to be patient not to open the cover for 7-8 minutes to make them juicy. If you open the cover and check how they got grilled by poking or shoving them, you'll miss the juicy part. This pan is roughly 28cms in diameter but the hambergers are a bit big so I was not able to put more.
Displayed it kinda rough. I use thin spaghetti e.g. Audesio No.5 or Barilla No.3 and boiled some with salty hot water to get them "Aldente" which means a little bit crisp. Tomato sauce is made from tomatoes we harvested in summer.