Melt the butter in a pot and saute the chopped onions for a few minutes until softened and translucent. While they cook, season the onions well with salt and freshly ground black pepper.
Once the onions are softened, add the carrots, potatoes and zucchini to the pot and season again lightly.
Pour in enough stock just to cover the veggies. You can always add more stock later on, but the stick blender works better at blending if your soup is on the thicker side.
Bring the soup to boil, then reduce the heat to low and let the soup simmer for about 15 minutes until the carrots are tender.
Remove the soup from the heat and blend it straight in the pot with a stick blender.
Stir the crème fraiche into the soup or pour the soup into serving bowls and then drizzle about a tablespoon of crème fraiche over each bowl.
Serve either hot or warm, garnished with some crème fraiche, olive oil, fresh herbs or even some shredded cheddar cheese.