3/4 pound small to medium Brussels sprouts, trimmed (about 2½ cups)
6 ounces pearl onions, peeled and trimmed (a generous 1½ cups)
2 tablespoons unsalted butter
2 tablespoons all-purpose flour
1 cup milk, Basic Chicken Stock, or commercial chicken broth, or ½ cup milk plus ½ cup chicken stock
1 teaspoon kosher salt
Pinch of cayenne pepper
2 tablespoons chopped fresh parsley (optional)
Put the Brussels sprouts and onions in the top of a two-part steamer or on a steaming rack or a plate in a bamboo steamer. Fill the bottom of the steamer pan or a pan large enough to hold the bamboo or steaming rack with water; the water should not touch the rack. Bring to a boil over medium heat. Place the vegetables on top, cover and steam for 10 to 11 minutes, or until a knife easily slips into the flesh. Set aside.
Melt the butter in a medium saucepan. Add the flour and cook over low heat for 5 minutes, stirring. Slowly add the milk, whisking briskly to prevent lumps; be sure to scrape the corners, where lumps can hide. Add the salt and cayenne. Increase the heat to medium. Cook for several minutes, stirring occasionally, until thickened.
Add the Brussels sprouts, onions and optional parsley. Cook over low heat until fully heated through.