01 - Melt the butter in a medium saucepan over medium heat. Add the finely chopped shallot or onion and cook, stirring occasionally, until soft and translucent, about 2 to 3 minutes.
02 - Sprinkle the flour over the softened aromatics and stir continuously for 1 minute to form a smooth roux and cook out the raw flour taste.
03 - Gradually pour in the whole milk (and heavy cream, if using) while whisking constantly to prevent lumps from forming. Continue cooking and whisking until the sauce begins to thicken, about 3 to 4 minutes.
04 - Fold in the green peas, salt, pepper, and nutmeg. Reduce the heat to medium-low and let the mixture simmer gently for 4 to 5 minutes, until the peas are tender and coated in a creamy sauce. Taste and adjust the seasoning as needed.
05 - While the peas simmer, toast the bread slices in a toaster or under the oven broiler until golden brown and crisp on both sides.
06 - Ladle the creamed peas generously over each slice of toast. Finish with a scattering of chopped fresh parsley and an extra crack of black pepper, if desired. Serve immediately while hot.