Scrub the potatoes and place in a large, heavy saucepan. Add water to just cover and a big pinch of salt. Bring to a boil over high heat. Reduce the heat to medium-low, partially cover, and simmer for 5 minutes.
Trim and peel the onions. If using boiling onion, cut them in half. If using 1 medium-large onion, cut into 1-inch pieces and separate the layers. Add onion to the boiling pot of potatoes, cover, and simmer for 5 to 7 minutes, until the onions and potatoes are tender.
Add the peas, increase the heat slightly, cover, and cook for 2 to 3 minutes, until the peas are just tender. Drain.
In the same pot, over low heat, melt the butter. Add the cream and bring to a simmer. Add the vegetables to the cream mixture, season with salt and pepper, and cook, stirring gently, until just coated with the cream and butter.
Taste for seasoning and serve hot in bowls.