Ideally, use fresh peas and fresh baby onions, Frozen peas are an acceptable substitute if fresh ones aren't available, but frozen onions tend to be insipid and are not worth using.
Alternatively, use the white parts of spring onions.

175 g/ 6 oz baby onions
15 g/ ? oz/1 tbsp butter
900 g/ 2 lb fresh peas (abut 350 g/12 oz shelled or frozen)
150 ml / ? pint/ 2/3 cup double cream
15 g/ 1/2 oz /2 tbsp plain flour
10 ml/2 tsp chopped fresh parsley
15-30 ml/1-2 tbsp lemon juice (optional)
Salt and freshly ground black pepper
Serves 4
Peel the onions and halve them if necessary. Melt the butter in a flameproof casserole and fry the onions for 5-6 minutes over a moderate heat, until they begin to be flecked with brown.
Add the peas and stir-fry for a few minutes. Add 120 ml / 4 fl oz/ ? cup water and bring to the boil. Partially cover and simmer for about 10 minutes, until the peas and onions are tender. There should be a thin layer of water on the base of the pan - add a little more water if necessary or, if there is too much liquid, remove the lid and increase the heat until the liquid is reduced.
Using a small whisk, blend the cream with the flour. Remove the pan from the heat and stir in the combined cream and flour and chopped parsley. Season with salt and pepper.
Cook over a gentle heat for 3-4 minutes, until the sauce is thick. Taste and adjust the seasoning; add a little lemon juice to sharpen, if desired.