Add barley, water and salt to a pot and bring to a boil. Reduce heat to low and simmer for 45-50 minutes until tender
In a separate pan, heat walnut oil and brown onions and mushrooms. Add garlic and lower heat to low. Cook down mushroom mixture over low heat for 15 minutes
Add the extra teaspoon of walnut oil (and tablespoons of beef broth) as needed to keep from sticking. Once the barley has cooked, add to pan with mushrooms. Add the rosemary and thyme. Stir to combine
Add remaining 1/2 cup of beef broth and mascarpone cheese and stir constantly until fully melted and bubbling