Preheat your slow cooker on a sautéing function if you have it available. Otherwise, don't worry about it.
Prep the vegetables, chop the leek, dice the carrots and slice the celery. If you wish to chop all this in the Thermomix you can put it all in (chopped into large pieces) and chop 5 sec/speed 6. I don't like chopping celery in the Thermomix as you end up with stringy bits, but you do you!
Place all of these into the slow cooker with the butter and sauté for a few minutes until fragrant and softened.
Add all the other soup ingredients (including the whole chicken) and cook on low for up to 6 hours or until the chicken is falling off the bones and the house smells amazing! You can add up to another 2 cups of water as you see it is needed, but this will depend a lot on the volume of your slow cooker.
When the chicken is cooked, remove as many of the bones as you can, but it is not essential. I like to do this for aesthetics more than anything. You could also use chicken thigh fillets instead of a whole chicken if you wish.
Shred the chicken with a fork as much as you can.
Make the roux: Place all ingredients into the Thermomix bowl and cook 5 min/Varoma/speed 3. It will not be thick at this stage. Pour this into the slow cooker and stir quickly to combine.
Cook for a further 30 minutes.
Serve with fresh tarragon and spinach leaves stirred through until it softens.
You can also serve with a swirl of cream if you wish.