Step 1Heat a 4-quart saucepan over medium-high heat. Add the butter and cook until it bubbles and turns brown on the edges. Add the onion and cook until tender and golden, about 5 minutes.
Step 2Add the couscous and cook until hot and starting to toast, about 5 minutes, stirring often. Season with salt and pepper. Add 2 cups of the stock or water and cook until the liquid is absorbed, stirring as you would risotto. Continue to add liquid slowly, waiting for the couscous to absorb the liquid before adding more, stirring regularly. Cook until the couscous is tender, about 40 minutes.
Step 3Remove the pan from the heat and fold in the parsley, tarragon and chervil. Season to taste with salt and pepper.