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 the vegetables with butter.
Let the butter slowly come to a sizzle, then cover the pot and cook for 5 minutes.
Add the mineral water and potatoes, stir, and let it come to a low simmer. Cover and cook for another 15 minutes.
Remove the thyme. Purée the soup using a strainer or immersion blender, then pass it through a fine sieve.
Return the puréed soup to a pot, add the cream and 150 g of butter, and stir until melted.
Melt butter or ghee in a pan, toss in the bread cubes, and toast until golden brown.
Ladle the soup into bowls, top with croutons and chervil, parsley, or cilantro, and serve.
