Melt the butter in a nonstick skillet. Add the onion and cook over moderately high heat, stirring, until browned, about 7 minutes. Add the stock and bring to a simmer. Pour over the fennel, cover tightly with foil and bake for 10 minutes. Uncover and bake for about 30 minutes longer, or until the fennel is tender. Mix in the olives, sun-dried tomatoes and basil and serve.