Melt the butter in a saucepan over a medium heat.
Add the leek and scallions and cook for 6 minutes, stirring occasionally until translucent.
Add the thyme and chicken stock, bring to a boil and then simmer for 20 minutes (the stock should be thick and reduced by half).
Season with salt.
Add the peas and simmer for about 4 minutes (the peas should be tender but still bright green).
Remove the thyme. and then puree the mixture in a blender.
Strain the mixture through a fine-mesh strainer.
Thin the sauce with chicken stock if needed (the sauce should be just thicker than heavy cream).
Stir the mixture until it is cool (in a bowl over ice if possible) and store in the fridge.
Your pea sauce is now ready to enjoy over a juicy steak!