Place the asparagus on a roasting tin, baking tray or swiss roll tin (use two if needed to prevent over crowding the tray). Drizzle with half of the olive oil, sprinkle with salt and pepper, then toss to coat. Place underneath the grill for 5 to 10 minutes, until asparagus is tender but still crisp (toss again halfway through grilling for even cooking). It's okay if the asparagus takes some colour, but don't let it get too brown. Remove from the grill and set aside.
Heat the remaining oil in a 4-litre pan over medium heat. Add the onion and garlic and cook until soft.
Once the asparagus is cool, cut into 2.5cm pieces. Set aside.
Add the flour to the onion mixture and reduce the heat to low. Cook for 5 minutes, stirring often. Gradually stir in the stock, then water. Bring to the boil, then cover and reduce heat to a simmer. Cook at a gentle simmer for 10 minutes. Add all of the asparagus pieces except for the tips.
Place 1/3 of the mixture into an electric blender or food processor container. Cover and blend until smooth. Pour the mixture into a medium bowl. Repeat the blending process twice more with the remaining mixture. (Or use an immersion blender right in the pan for ease.)
Return all of the pureed mixture to the saucepan. Add the remaining asparagus tips, chives and lemon thyme. Adjust seasoning with salt and pepper as desired. Cook over medium heat until the mixture is hot. Ladle into bowls and serve with a spoon of creme fraiche, if desired.