Melt butter in a large saucepan over medium heat. Add onion and garlic. Cook, stirring frequently, until slightly softened, about 3 minutes. Add stock, potatoes, leeks, parsley, bouquet garni, bouillon cube, salt, and pepper. Bring to a boil. Reduce heat and simmer until potatoes are tender, about 30 minutes.
Remove soup from heat; discard bouquet garni. Puree soup in the pot using a hand blender.