Prepare your ingredients. Peel two medium potatoes and chop them into bite-size chunks, peel and dice one medium carrot. Remove the onion skin and finely dice it. Rinse then slice the mushrooms and chop the celery.
In a large deep pot warm up two tablespoons of olive oil. Add the onions, carrots and celery then sauté for 5 minutes
Add mushroom (about 5 oz sliced) and a tablespoon of minced garlic (this could be about 4 to 6 garlic cloves, depending on their size) and season with 1 teaspoon of ground cumin then sauté for 3 minutes
Add ¾ cup of barleys and ¾ cup of red lentils or green. Add the potatoes as well followed by 4 cups of vegetable broth and 1 cup of tap water. Bring it to a boil then let everything simmer on medium heat for 30 to 35 minutes.
Do a taste test and adjust seasoning as needed, mostly a pinch of salt and a sprinkle of some pepper is all you need.
If the potatoes are cooked and the barley is cooked as well as the lentils then your barley lentil soup is ready to be served. Bear in mind that barleys are chewy by nature, I hope you don't confuse that with them being undercooked