I used to do this when mine were babies.

I would use chinese containers or tupperware. You could do it a week before but you'd want to make sure you stored it correctly. The tupperware fridgemates are good for this.

You can chop and freeze onions most definitely. Just like you can make your own grated cheese by grating it all and adding a sprinkling of corn flour to stop it from sticking (or you can snap freeze grated cheese on a tray and then pop in a bag).

When I was time poor because of babies I did a lot of this stuff. We basically lived off slow cooked meals and easy cook meals.