Mushy peas are very traditional in the north of England. They go brilliantly with fish and chips, pies, etc. But people who are not initiated into the craft have this idea that mashed garden peas will do as a substitute. They are massively wrong - this is how you make mushy peas, there's no other way.
About the Peas
Making proper mushy peas is all about having the right peas to start with. In the UK, they are dried marrowfat peas. In the US, they are known as field peas. They are perfectly normal peas that have been left in the field to mature, rather than being picked early for freezing.
Soak the Marrowfat Peas
The peas need to be soaked before they become cookable. Most recipes say overnight, and that's fine, but if you're a non-planner like me, 3-4 hours does it. Put the peas in a bowl and cover with plenty of hot water (the peas will grow to 2-3 times their dried size). Add a teaspoon of bicarbonate of soda and stir it in.
Cook the Mushy Peas
Drain the soaked peas and rinse them well to get rid of any bicarb. Put in a pan and add enough water to just cover them. Bring them to the boil. You might find that the skins come off the peas - don't worry about this, just stir them in and they'll disappear during cooking. Turn the heat down to a simmer and cover with a lid. Cook for 30 minutes, then remove the lid. The peas will have started to disintegrate and the cooking water will be a thickish green. The ideal consistency for mushy peas is like a thick paint, but with lumps of peas in it. If it is too dry, add a bit more water. If too wet, increase the heat to reduce some of the liquid.
When you've achieved the right consistency, add salt until it tastes right.
Some people add sugar and/or mint to mushy peas. Me, I despair. Not needed.