I did omit the instant minced onion and as I have an aversion to fresh breadcrumbs in my meatballs or meatloaf (don't like having those little bit of bread in the mix) I toasted my wholemeal/wholegrain bread to dry it out and then processed in the food processor to a fine crumb and used that with a very low fat mince and consequently really only needed half the amount along with the egg and salt and pepper to get the texture I thought was right. I got 13 meatballs (50 grams each) which was more than enough for 3 of us (had 4 left over) and I used up what cheeses I had on hand (Monterey jack is not available to us) which was mozzarella for the meatballs and a mild cheddar to top the dish and I also used a Jensen's Organic Basil and Tomato sauce for my spaghetti sauce and cooked as directed but just allowed some extra time on the meatballs as I had made them bigger than specified but we thoroughly enjoyed the gooiness of the mozzarella cheese and the sauce that flavoured the spaghetti, thank you keen5, made for Name that Ingredient tag game.
Had to change the method out of necessity...I didn't have any 'hard cheese' to cut up into cubes and use for the center, so I just added 1/2 cup of PIZZA flavored shredded cheese into the meat. (BTW I used ground turkey instead of ground beef.) I did SEAR the meatballs in a TBS of oil in a big HOT saucepan first and swirled and turned them in the pan until all sides were browned, then I transferred them to a 13x9x2 pan (sprayed with PAM). I added 1/4 cup of herbed water to the bottom, covered with foil, and steam/baked @ 350 for 25 minutes. We ate these with more shredded pizza cheese on top and a dipped them in ketchup. HELLO! Very yummy! They would have been JUST as good with spaghetti and sauce but we couldn't wait. My meatballs were on the large side (about 2"). Thanks for the basic idea keen5, you helped me get creative!!