Cover potatoes with cold salted water in a large pot. Bring to a boil and cook until fork-tender, 10–12 minutes.
Drain potatoes well and set aside. Keep warm.
Melt butter in a wide saucepan over medium heat. Soften onion until translucent, 4–5 minutes.
Stir in garlic and cook just until fragrant, about 30 seconds.
Sprinkle in flour and whisk to form a paste. Cook the roux 1–2 minutes, stirring.
Slowly whisk in warm milk, then cream, until smooth. Simmer, whisking, until thick enough to coat a spoon, 3–5 minutes.
Fold in peas and simmer until bright and tender, 2–3 minutes.
Add potatoes and gently toss to coat. Season with salt, pepper, and nutmeg. Sprinkle with parsley and serve.