Bring a pot of salted water to a boil. Nearby, have a large bowl of ice water ready.
Trim the haricots verts into 3-inch pieces and blanch them in boiling water for about 1 ½ to 2 minutes.
Transfer the beans with a slotted spoon to the ice water to immediately stop the cooking process. Drain them in a colander.
In the same boiling water, cook the onions for about 5 minutes and transfer to a colander.
Peel and halve the onions and transfer them to a bowl.
The vegetables may be prepared a day ahead of time up until this point. Make sure they are cooled completely, and chilled in a sealable plastic bag.
Cut the chanterelles in half or quarters, if they are large. In a heavy skillet, heat the unsalted butter over moderate heat until the foam subsides. Cook the mushrooms with salt to taste, stirring occasionally, for about 2 or 3 minutes, or until they are tender.
Add the peas and cook another 2 minutes, or until they are a bright green.
Add the beans, onions, and potatoes, and cook, stirring occasionally, until they are just heated through.
Stir in the chives and season with salt and pepper.