Drain the asparagus; save the liquid from the can.
Take half a cup of the liquid and pour into a bowl. Add the miso paste to the liquid in the bowl and stir until smooth.
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.
Stir in the dashi.
Turn off the heat. Stir in the miso.
Serve the white asparagus and spinach soup immediately.