Place the potatoes in a medium saucepan. Cover with salted water and bring to a boil. Reduce the heat and simmer until the potatoes are tender. Drain well and return to the pan.

Meanwhile, in a small saucepan, melt the butter over medium-high heat. Add the green onions and garlic and cook, stirring, until soft and fragrant, 1 1/2 minutes. Remove from the heat.

Return the pan with the potatoes to medium-low heat and cook for 1 minute, shaking the pan. While mashing with a potato masher, slowly add the milk, mashing until the potatoes are creamy. Add 1/2 of the hot butter with the green onions and mash to incorporate. Adjust seasoning with salt and pepper, to taste.

Remove from the heat and fold in the cooked peas. Transfer to a serving bowl and drizzle the remaining butter and onions over the top. Serve hot.