Thoroughly wash the spinach and remove any long stems.
Season the sliced shiitake caps with ¼ tsp of salt.
In a large skillet, heat 2 tbsp cooking oil to medium-high.
Add the onions and fry for 2-3 minutes until they start to darken.
Add the garlic and mushrooms and continue to fry, stirring often, until the onions are caramelized on the edges.
Stir in 20g butter and spinach.
Keep stirring until all leaves wilt and immediately remove from the heat.
Season with salt and pepper to taste and serve warm.
