Place the barley, lentils, onion and vegetable stock in a 4-quart slow cooker. Set the cooker on HIGH.
Put the dried mushrooms and 1 cup of water in a glass measuring cup. Microwave for 1-1/2 minutes. Remove from the microwave and set aside to cool for 30 minutes. Gently scoop the mushrooms out of the measuring cup, being careful not to disturb the sediment at the bottom. Finely chop the mushrooms.
Add the mushrooms to the slow cooker with the tomato paste, thyme, one teaspoon each of salt and pepper, and the fresh mushrooms. Pour in the mushroom soaking liquid, being careful not to include the sediment.
After the cooker has been on for 4 hours, the lentils and barley should be done. Turn off the cooker, and stir in the spinach; the heat of the pilaf will soften the spinach. Adjust seasoning with salt and pepper as needed (and you will definitely need it). Serve hot.