Melt the butter in a medium saucepan and cook the onions and lardons for about 5mins over a low heat until the onions have started to soften.
Stir in the flour and cook for another minute, turn the heat up to medium and then gradually add the stock and bring to a boil.
Add the peas then turn the heat back down to low, cover and cook for 15-20 mins. Note: if the sauce looks like it going a little dry, add a little bit of water.
When cooked, the sauce will be slightly thick. Season with sugar, salt and pepper and serve immediately.