chopped onions -> put in airtight bag and freeze it. Or you can do the same as for garlic...
chopped tomatoes -> buy canned chopped tomatoes. If you are going to cook them, it's the best ones you can use. (Not so good for salads and on sandwich...

)
chopped bell pepper (capsicum) -> freeze it.
Vegetables (cauliflower, broccoli, carrot, beans, spinach etc) -> cut in to pieces ready to be cooked -> blanche them and put them into freezer bags and store in the freezer. Can last up to 12 months! Easy to use too, just take out as much as you need and dump right into the pan. No need to defrost!
side note:
I find the freezer to be a sorely underutilized item in this country. Sure things are available in the market, but buying in big bulk can save a lot of money and time, without losing out on the freshness!! And it is not always the items you want are fresh in the market....