Bring 1 onion, cut in half, skin and stems still on to a boil in a small dutch oven with stems of parsley, stems of basil, 1 1/2 tsp kosher salt and 6 cups water. Gently simmer for 30-45 minutes.

Strain the liquid into a bowl with a mesh strainer over the top to catch the spent onion and herbs. Set the bowl aside. Keep the broth but discard the items in the strainer.

Wipe the pot clean. Put it over medium heat and melt the butter. Add the other onion, diced, and the asparagus, cut into 1 inch pieces. Sprinkle with 1/4 tsp kosher salt and saute for 5-8 minutes until onion is soft and asparagus is still bright green.

Add broth back to the pot along with the peas, lemon zest, and basil leaves. Pull the pot off the heat and mix in the creme fraiche.

Allow to cool for 10 minutes. Blend in a blender or food processor 1/3 of the soup at a time, making sure to place a hand towel over the lid to ensure nothing splatters out. Transfer to a serving bowl and serve warm or chill for at least 4 hours.

Serve as is with cream fraiche dolloped over the top, or drizzled with a little olive oil and crack of black pepper.