Heat olive oil in a large pot over medium heat. Cook onion for 2 minutes, until softened, then stir in celery and cook for 2 more minutes.
Add potato, season dish with salt, and continue to cook, stirring, for 5 minutes.
Add sausage, water and thyme and simmer for 15 minutes, until potato is soft enough to be pierced by a fork with still some resistance.
Gently stir in cod and heavy cream and cook for 5 minutes.
Add kale and cook for about 3 minutes, until softened but retaining its color. Remove from heat. Stir in Dijon mustard and season to taste with salt and pepper. Enjoy while hot.