Peel and cut the potatoes into chunks. Place in a saucepan, cover with water, and boil until tender, about 20 minutes.

Melt butter in a small skillet and fry onions over medium-low until tender and slightly browned. Add peas and cook until heated through. Remove from heat and set aside.

Drain the potatoes, place them in a medium bowl along with the cheese, and mash with a fork.

Mix the peas and onions into the cheesy potatoes. Season with salt and pepper to taste.