We have always made our own marrowfat mushies. Soak the peas overnight in cold water and some bicarb. Next day, drain off all the water and rinse. Add some fresh water (salted if you wish), bring to the boil, then simmer until soft. Never let them boil over or they will split open, just a gentle simmer. We tend to remove them from the hob before they are fully cooked and decant the whole lot (including the hot water) to a vacuum flask to finish off cooking. They can then stay in the flask until you are ready to eat them - just heat up again in microwave or whatever. The vacuum flask is not necessary, but we find it convenient. Just make sure it is an old flask and not one you want to use on a picnic. Coffee tastes of peas.
Home made are a trillion times better than tinned, and also a million times better than chippy peas as sometimes sugar is added.
Portion size is individual - needs testing!
I am also fine with frozen garden peas, and happily a tablespoon of baked beans with fry ups. (The beans were not always fine. It is something I have been able to add back since normalising my blood sugars.