In a large pot, cook the onion in oil on medium heat until starting to soften, about 3 minutes. Then, add in the sliced mushrooms and cook until they start to brown and release their water, about 10 minutes. Try not to disturb them too much - just stir occasionally to make sure they are evenly browned.
Season the mushrooms with salt and pepper. Add the garlic, stirring constantly for a minute, then add the water and bouillon concentrate (or stock), cumin, and paprika and bring to a boil on high heat.
Add the dry lentils, let boil for 1 minute, then reduce heat to medium and cook, partially covered, until lentils are tender and the stew is thickened, about 18-25 minutes. Stir occasionally, you want the mixture bubbling, and if the stew is getting too thick before the lentils are cooked, add ½ cup water at a time.
Meanwhile, place red peppers, tahini and vinegar in a bullet blender or wide mouthed blending jar to use an immersion blender. Then blend until smooth, set aside.
When the stew is gravy-like in texture (if you need to, let cook with the lid off for a while) and lentils are tender, add ¼ cup of the hot stew to the pepper mixture, stir to temper and then add the mixture back into the stew, stirring until well combined.
Serve with a generous sprinkle of minced parsley, bread, rice or mashed potatoes!