Bring water to a boil in a large saucepan. Add onions and wakame and cook for 2-3 minutes to soften. Lower heat to a simmer and whisk in miso paste, making sure it completely dissolves. Chop tofu into bite-sized pieces (whatever that means to you) and add to saucepan. Serve miso soup while it is still warm! Also keeps well in the fridge for about a week.