Saute the onion in butter in a sauce pan over medium heat until it's tender, adding salt as you do. Add the curry paste and stir to cook the curry paste, 30 seconds or so.
Cut the lemongrass stalk into four-inch pieces, then halve each one (discard root and ratty ends).
Add the galangal, lemongrass, lime leaves (reserve some for garnish if you wish, using at least four in the soup here), and chilies. Stir to heat and bring out their fragrance. Add the chicken stock. Bring the soup to a simmer, then reduce the heat to low, barely at a simmer. Cook for 45-60 minutes. (If you think the stock has cooked down too much, add a cup of water.)
Strain the stock into a clean sauce pan (discarding the onion-lemongrass aromatics). Add the coconut milk, the chicken and mushrooms, bring to a simmer for a few minutes . Add lemon juice and fish sauce to season the soup (a tablespoon of lime, 2 teaspoons of fish sauce, around there, again to taste). Serve alongside cooked jasmine rice.