Over medium high heat sauté olive oil, onion and garlic until onions are translucent. Add mushrooms, salt and pepper and sauté for 10 minutes or until mushrooms are tender and have released their juices. Cook mushrooms for another 10 minutes or until juices are almost evaporated. Add Marsala wine and cook 3-4
minutes stirring to deglaze pan and reduce Marsala slightly.
To make the roux, in separate sauce pan melt butter over medium heat. Once the butter is hot enough that the flour sprinkled into it will slowly start to bubble, then slowly whisk in the flour. Continue to cook and whisk butter and flour for 3-4 minutes to remove starchy flavor of flour. Slowly whisk in vegetable broth. Simmer while whisking for 3 minutes until creamy. Whisk in tamari and lemon juice.
Add roux to cooked mushrooms. Stir in parsley.