Wrap your tofu in paper towels and lay on a plate. Compress the tofu under another plate with a weight and leave for 20 minutes. After compression, dice the tofu. If you want more flavor in your tofu, marinate it in a few dashes of soy sauce. Heat the dashi in a saucepan with the wakame and shitakes (if desired). Add the miso and wisk until dissolved. Bring to a simmer and add the tofu and onions. Cook for a minute more to heat through and serve.