1 Tbsp. vegan butter
1/2 cup vegan vegetable or vegan beef broth
1/2 small yellow onion, diced
1 large garlic clove, minced
3 Tbsp. balsamic vinegar
1 Tbsp. mirin or sherry
½ Tbsp. soy sauce
½ Tbsp. tomato paste
1 tsp. dried thyme
½ tsp. dried basil
dash of ground black pepper
2 large, whole Portobello mushrooms"
Directions @ One Green Planet