4 tablespoons butter
1 (16 ounce) package frozen pearl onions or small whole white onions
2 pounds green beans, trimmed and snapped into 2-inch pieces
1 1/2 cups milk
1/2 cup chicken broth
1/4 cup Dijon mustard
3 tablespoons flour
Freshly ground black pepper
1/2 cup slivered almonds, toasted until fragrant in a 325-degree oven (10 minutes or less)
Heat 2 Tbs. of butter in a large, deep skillet over medium-high heat.
Add onions; saute, shaking pan occasionally, until golden brown, 5 to 7 minutes.
Dump onions onto a large cookie sheet. Add green beans, 3/4 cup water and a scant teaspoon salt to the unwashed skillet. Turn heat to high; cover and cook until wisps of steam escape around the lid.
Set timer for 5 minutes; cook until bright green but tender.
Drain beans; transfer to cookie sheet with onions and spread out to cool. (Cooled vegetables can be covered loosely with plastic wrap and set aside at room temperature up to 4 hours ahead.)
Microwave milk, chicken broth and mustard in a 1-quart Pyrex-type measuring cup until steamy. Heat remaining 2 Tbs. of butter in the skillet. Whisk in flour, then hot milk mixture, until smooth and simmering. (Sauce can be made up to 4 hours ahead: Cover surface directly with plastic wrap, to prevent a skin from forming, and refrigerate; return to a simmer before proceeding.)
Add beans and onions; simmer until sauce is thick enough to cling, about 5 minutes. Season with salt and pepper to taste. Transfer to a serving bowl, sprinkle with toasted almonds and serve immediately.