Drain the mushrooms; save the liquid from the can.
Add enough liquid from the asparagus to measure four cups. Pour into a pot and heat.
Add the spinach leaves and asparagus spears to the pot. Cook for a minute after the water boils.
Turn off the heat. Take half a cup of the liquid and pour into a bowl.
To the remaining liquid in the pot, stir in the dashi.
Add the miso paste to the liquid in the bowl and stir until smooth. Stir into the contents of the pot.
Serve the white asparagus and spinach soup immediately.