Melt 175g/7oz of the butter in a large sauté pan and gently sweat the onions for 6-8 minutes until softened and caramelised.
Bring a pan of salted water to the boil and blanch the sprouts for 1 minute. Drain and refresh in ice cold water.
Heat the oil in a sauté pan and add the almonds. Fry gently for a few minutes until lightly toasted.
Melt the remaining 25g/1oz of butter in a large sauté pan and sweat the sprouts for 3-4 minutes, tossing, until just softened. Stir the onions and almonds through. Season with celery salt and lots of pepper. Serve at once.