In a pan, sauté the shiitake and bella mushrooms over medium-high heat until golden brown.
Add the thinly sliced onion and cook until translucent.
Transfer the mushrooms and onions to a saucepot. Add enough water to cover by about 1 inch.
Add bay leaf, thyme, and a pinch of black pepper. Simmer for 30–45 minutes to develop a mushroom stock.
Strain the mixture, removing the bay leaf and any stems. You can reserve the flavorful mushroom stock for use in a different recipe later.
Transfer the cooked mushrooms and onions to a blender.
Add olive oil and the mushroom stock a little at a time, blending until smooth and the desired sauce consistency is reached.
Season with salt to taste.