Place potatoes in a large pot with enough water to cover by an inch. Bring the water to a boil, cover with the lid, lower the heat, and cook another 10 minutes or until a knife pierced in the center of the potato goes in cleanly.
Drain the potatoes immediately and set aside. Place the frozen peas in the warm pot you cooked the potatoes in and cover. This should be enough to warm them without overcooking them.
To make the dressing, heat the oil in a small saucepan. Add the mustard seeds. When they crackle, add the turmeric, red pepper flakes, and ginger. Stir-fry for 30 seconds, then turn off heat. Let it cool for a few minutes, then stir in the coconut milk.
Halve the potatoes and place in a large bowl. Add in the green peas, the turmeric-coconut dressing, and the spring onions. Squeeze in the lemon juice. Add salt to taste and toss everything together.
Serve warm or cold.