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 potato cooking. Drain well and set aside.
03 - Melt butter in a large skillet over medium heat. Whisk in flour and cook, stirring constantly, for 1–2 minutes until roux forms and bubbles slightly.
04 - Gradually whisk in milk, followed by heavy cream. Continue stirring until sauce is smooth and begins to thicken, 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. Cook for 2 minutes until heated through.
07 - Transfer to a serving bowl and garnish with fresh chopped parsley.