01 - Place halved potatoes in a large saucepan and cover with cold salted water. Bring to a boil over medium-high heat. Reduce heat and simmer for 12–15 minutes until fork-tender.
02 - Add peas to the pot during the last 3 minutes of cooking. Drain and set aside.
03 - Melt butter in a large skillet over medium heat. Whisk in flour and cook, stirring constantly, for 1–2 minutes to form a smooth roux.
04 - Gradually whisk in milk followed by heavy cream. Stir until smooth and thickened, about 3–5 minutes.
05 - Add salt, pepper, and nutmeg if using. Stir to combine.
06 - Add drained potatoes and peas to the sauce. Gently fold to coat evenly and cook for 2 minutes to heat through. Transfer to a serving bowl and garnish with fresh parsley.