Preheat the oven to 375 degrees and line a baking tray with parchment paper.
Wash mushrooms and dry really well (mushrooms absorb water) with a cloth.
Remove mushroom stems by lightly tugging them off.
Mince the stems into very small pieces. Then mince the garlic and chop the walnuts.
Heat oil in a medium pan over low to medium heat. Then add the mushroom stems and saute for 2-4 minutes until they soften.
Add garlic to the pan and saute for a few more minutes until garlic becomes fragrant.
Add walnuts, spinach, water, salt, and pepper to the pan and saute until the spinach wilts. Turn heat off.
Place the mushrooms on a baking tray and scoop the spinach mixture into the mushrooms filling them up to the top. Continue until all mushrooms are filled up!
Place the mushrooms in the oven (bottom rack) and cook for 15 minutes. Remove from the oven and ENJOY!