Shell the peas, reserving about 12 pods, which you should wash thoroughly and top and tail.
Melt the butter in a saucepan, tip in the onion and gently fry for about 10 minutes. Throw in the peas, the 12 pods and the savory. Stir, then pour in 750ml/11⁄4 pints boiling water and bring to the boil. Reduce to a gentle simmer, cover and cook for 8-10 minutes until the peas are tender but still bright green.
Tip half the contents of the pan into a blender or food processor and whizz to a purée. Push the purée through a fine sieve into a clean pan (to eliminate stringy bits), then repeat with the other half of the soup. Stir and season to taste.
Gently reheat the soup, check the seasoning once more and serve with or without a swirl of cream.