Slice off the root ends of the pearl onions. Drop them into boiling water for 2 minutes to loosen the skins. Drain and rinse the onions under cold water. Pinch the stem end of each onion to pop them out of their skins. Set the peeled onions aside for later use.

In a large skillet over medium-low heat, melt the unsalted butter. Whisk in the flour and keep whisking until the mixture turns a light golden brown, about 2 minutes. Gradually whisk in the room temperature whole milk. Continue whisking until the sauce thickens slightly, which should take around 2-3 minutes.

Add the frozen peas and the peeled, halved pearl onions from Step 1 into the skillet with the thickened sauce. Let the mixture simmer gently for 5-7 minutes, or until the peas are tender. Season with kosher salt and freshly ground black pepper to taste. If you like, stir in a pinch or two of ground nutmeg and a splash of heavy cream for extra richness—I always recommend adding the cream if you want a luxurious finish.