In a large saucepan, heat the olive oil over medium heat. Add the onions and cook, stirring often, until tender, about 7 minutes. Add the potatoes and fish stock and bring to a boil. Lower the heat and simmer until the potatoes are tender, about 7 minutes. Stir in the edamame and thyme and keep warm over low heat.
Meanwhile, in a small saucepan, melt the butter over medium heat. Whisk in the flour and cook for 1 minute, then whisk in 1/2 cup milk until thickened. Whisk the cornstarch into the remaining 1/2 cup milk, then whisk into the hot milk mixture; stir in the cream. Cook, stirring often, until thickened and steaming, about 3 minutes.
Stir the cream mixture into the potato-broth mixture; add the clams with their liquid. Bring to a simmer and cook for 5 minutes more; season with salt and pepper.