Dice the potatoes into even pieces. Peel them if desired. Thaw the frozen peas.
Add the potatoes to a pot of cold water with 2 teaspoons salt. Bring to a boil and cook for about 15–20 minutes, or until fork tender. Drain well.
Melt the butter in a large pot or saucepan over medium heat.
Add the garlic and cook until fragrant and lightly golden.
Stir in the flour and cook for about 30 seconds to 1 minute, just long enough to remove the raw flour taste.
Gradually pour in the whole milk while whisking.
Bring the sauce to a gentle simmer and allow it to begin thickening. Do not bring it to a hard boil.
Add the drained potatoes and thawed peas.
Simmer gently for about 5 minutes, or until the peas are heated through and the sauce coats the vegetables.
Stir in the heavy cream and heat through without boiling.
Remove from the heat, then season with salt and black pepper to taste.