Put the potatoes in a large saucepan with cold water to cover by 3 inches. Salt the water, cover and bring to a boil over high heat. Reduce heat and simmer until tender, about 10 minutes. While the potatoes are cooking, cook the peas in a separate saucepan of salted water. Simmer until just tender, about 3 to 5 minutes. Remove with a sieve and cool on a baking sheet. When the potatoes are done, drain well, scatter on a baking sheet, and let cool for several minutes. In a small saucepan, heat 1/2 cup of the cream until it reaches a boil then simmer for a few minutes. Add the cream to a blender with the cooked peas and puree until smooth. Season with salt and pepper.
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 remaining cream, season, to taste, with salt and pepper and bring to a boil.
Press the cooled potatoes through a ricer or food mill into the butter and garlic mixture, tossing to coat.
Reduce the heat to low. 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, if desired.