Step 1Place the spinach in a microwave-safe bowl, cover with plastic wrap and cook until just wilted, about 4 minutes. Set aside.
Step 2Bring a saucepan of water to a simmer. Meanwhile, melt 3 tablespoons of butter in a large saucepan over medium heat. Add the onion and cook, stirring until softened, about 3 to 4 minutes. Sprinkle the flour over the onion and cook another 2 minutes, until the flour has lost its raw taste. Add the broth, milk, salt and pepper to taste and the nutmeg. Cook, stirring, about 3 minutes. Stir in the spinach and cook just to heat through, about 2 minutes. Keep warm.
Step 3Melt the remaining 1 tablespoon of butter in a small skillet over medium-high heat. Add the bread crumbs and cook, stirring, until golden and crisp, about 2 minutes. Set aside while you poach the eggs.
Step 4Add the vinegar to the simmering water, then break 1 egg into a small bowl and slide it into the water. Repeat with another egg. Cook the eggs, 2 at a time, until the whites are set and the yolks look slightly pink, 3 minutes. Remove the eggs with a slotted spoon.
Step 5To assemble, place 1 piece of toast on each of 4 plates. Divide the spinach on top of each slice, then top with a poached egg. Sprinkle with the bread crumbs.