Wild Mushroom Gravy

1 tbsp. olive oil

1/2 c. chopped onion

1 tsp. dried thyme

1/4 tsp. salt

8 oz. oyster mushrooms, sliced

1/4 c. dry sherry

3 c. mushroom stock or vegetable broth

1 1/2 tbsp. all-purpose flour

2 tbsp. water

2 tbsp. butter (or Earth Balance)

1/8 tsp. black pepper