In a large pot over medium-low heat, melt the 75 g of butter.
Add the onions, carrots, thyme, salt, and sugar. Stir to coat with butter.
Let the butter slowly come to a sizzle. Cook for 5 minutes with the lid on.
Add the mineral water and potatoes, stir, and let it come to a low simmer.
Cook for another 15 minutes, or until you can mash the potato easily with a fork.
Remove the thyme. Purée the soup using a strainer or immersion blender.
Pass the puréed soup through a fine sieve.
Return the soup to a pot, add the cream and the 150 g of butter, and stir until melted.
Melt butter or ghee in a pan for croutons. Toss in bread cubes and toast until golden brown.
Ladle the soup into bowls, top with croutons and chervil, parsley, or cilantro, and serve.
