Here is what I have done since my kids were young and wouldn't eat either onions, peppers or cooked carrots that they could see: I put them each raw into my food processor and mince finely.. or puree. Then I put into a gallon freezer bag and flatten and freeze. Then I can just break off the size piece of each I need for a certain recipe and it thaws quickly because it's thin. I use these in all kinds of things including chili, meatballs and meatloaf, soups, casseroles, etc. They add a lot of flavor and are not there for the kids to see.
When I decide what I am going to make, I break off the veggie chunks and set them out to thaw. If it's last minute, a short time in the microwave thaws them even quicker.