Heat the olive oil in a saucepan. Add the onion and sweat for five minutes.
Add the peas, water, stock cube and double cream, then reduce the heat and simmer gently for 3-4 minutes or until the peas are cooked.
In a small bowl, mix the butter and flour together to make a paste. Slowly add the paste to the peas a little at a time, whisking it in, until the liquid becomes thick enough to coat the back of a spoon.
Add the grated carrot and the tarragon, and season with salt and freshly ground black pepper. Serve in two soup bowls.