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Put the potatoes in a large saucepan with well-salted cold water to cover them by 3 inches. Cover and bring to a boil over high heat. Cook until tender, about 10 minutes. While the potatoes are cooking, add the peas to the pot. They will float on the top. Cook for 2 or 3 minutes, then remove with a sieve. Cool on a sheet tray.
The potatoes are done when you can easily insert a knife through the potato. When the potatoes are done drain well, scatter on a baking sheet, and let cool for several minutes. Place in the oven to dry out, about 5 minutes. Press the potatoes through a ricer or food mill or mash with a potato masher.
Heat 1 tablespoon of the butter in a large saucepan over medium heat until the butter stops foaming and turns a light brown. Add the garlic and cook quickly until pale gold. Add the 1 1/2 cups of the cream, season, to taste, with salt and pepper and bring to a boil. Reduce the heat to low and beat in the potatoes with a wooden spoon or large whisk. Add the remaining butter by tablespoonfuls, stirring vigorously after each addition. (In this way, the butter does not melt into a golden puddle. Instead, it forms an emulsion with the potatoes, creating a very smooth puree.)
Fold the pea puree into the mashed potatoes and adjust seasoning as necessary. Instead of salt, the puree can be seasoned with a little grated Parmesan. Serve hot.