Bring a pot of salted water to the boil and cook your spaghetti until al dente.
In the meantime, add a tsp or so of olive oil to a saucepan over medium heat.
Add your onions and garlic along with a pinch of salt and sauté for a few minutes until soft and translucent.
When your onions are soft, add your peas and chicken stock to the pan.
Simmer for 5-10 minutes until the peas are very tender and the water/stock has almost completely evaporated.
Add your basil leaves to the pan and give it a quick stir until they have just wilted.
Transfer everything to a blender or food processor along with the feta. Blend it all together until complete smooth.
If it's too thick and isn't blending, add the milk and puree again.
Taste the sauce then adjust with the sugar, salt and pepper until it's how you'd like it. The sugar shouldn't make it sweet, but instead just enhance the natural sweetness of the peas.
If your sauce seems a little grainy still, pass it through a sieve to remove any lumps.
When your pasta is cooked, pour your sauce into a pan over medium-high heat and stir until warm and just starting to bubble at the edges.
Transfer your pasta with tongs straight from the pot to the sauce and toss to coat, allowing most of the water to drip away. Don't worry if you've brought over some water with your pasta, this is a good thing as the starchy liquid will help to emulsify the sauce and coat the pasta.
Continue to toss together in the pan until the sauce has thickened and coats the pasta perfectly.
Transfer to a plate and top with some fresh basil and parmesan if you'd like and enjoy.