Set a large pot over medium heat.
Heat the olive oil for a minute, then add the yellow onion, green onion, and garlic to the pot, and sauté until they soften and just begin to take on a bit of colour.
Add the cauliflower and sauté for a couple of minutes, then add 4 cups of vegetable broth.
Bring to a boil, then reduce the heat to medium low and simmer, covered, until the cauliflower is tender - about 15 minutes.
Now add the peas, wait a minute for them to soften and then blend the soup (I used an immersion blender, but if you don't have one you could transfer to a blender in batches).
Once the soup is well blended, taste, and season with salt and pepper.
Return to the heat, and stir in the creme fraiche or Greek yoghurt.
If you want to add extra broth to thin out the soup, do it now.
Add the mint and the lemon zest, stir well, and taste again. Serve immediately, with a drizzle of yoghurt, or a bit of lemon zest for garnish.