To make the mashed potatoes, place the potatoes in a large saucepan and cover them with 2 inches of water.
Bring the potatoes to a boil over high heat and stir in the salt.
Reduce the heat and simmer until the potatoes are tender when stabbed with a fork (start checking them at 15 minutes).
Drain the potatoes, and return them to the pot.
Add the milk, butter, salt and pepper, to taste.
Gently mash the potatoes with a handheld potato masher.
Use a large spoon to skim any fat from the surface of the beef and veggie mixture in the slow cooker.
In a small bowl, mix the flour and 6 tablespoons water until smooth, then slowly stir the mixture into the slow cooker. Add the peas. Adjust the seasonings to taste with salt and pepper.
Mix the cheddar cheese into the mashed potatoes and spread in an even layer over the top of the beef and vegetables.
Cover the slow cooker and cook on high for 15 to 30 minutes, until the gravy has thickened and the mashed potatoes are heated through.