Chop all of the vegetables and (optional) meat before you begin, since this soup comes together quickly.
Open the can of coconut milk, and spoon out about 1/4 cup of the "cream"
Melt the cream in a heavy stock pot, then add the scallions, garlic, ginger, and curry paste
Stir until softened, about 5 minutes
If you are going to add chicken, stir it in for about 5 minutes, until cooked through
Add the rest of the coconut milk and the broth
Add the lime juice and the fish sauce (or tamari sauce)
Simmer together for about 5 minutes
If you are going to add shrimp, do so now
Add the sugar snap peas and mushrooms, simmer about 2 minutes
Add the tofu, if desired
Add the bok choy, and stir to heat.
Serve immediately, garnishing with cilantro, green onions, and lime wedges
If the soup is not all going to be eaten at one sitting, you may want to only add the amount of bok choy that will be eaten at once, reserving the remainder to be added when the  left-overs are served.