SAUTE. Heat a large skillet with 1 tablespoon neutral oil on medium-high heat. ADD onions and a generous pinch of salt and saute until translucent about 1-2 minutes, add garlic and saute until garlic becomes fragrant. ADD. Shiitake mushrooms, carrot, celery, red pepper, tomato paste and stir to combine. ADD. Vegetable stock and turn heat to medium-low and cover. Let cook stirring occasionally until mushrooms have released their liquid, vegetables have softened and most of the cooking liquid has absorbed, about 20 minutes. Remove lid and cook a few more minutes until all liquid is absorbed. Turn off heat set aside and let cool.