Place olive oil and 1 tablespoon butter in large stock pot over medium high heat.

Add mushrooms, onion, carrots, and garlic. Saute 10 minutes, or until tender.

Remove and reserve veggies and any juices from stock pot and turn heat down to medium.

Make roux: add remaining 2 tablespoons butter. When melted, whisk in flour. Cook and whisk over medium until golden and nutty smelling, about 5 minutes.

Slowly whisk 3 cups stock into roux, about 1/2 cup at a time, until smooth and bubbly.

When roux and stock are incorporated, add rice, remaining stock, reserved veggies and their juices, and salt and pepper.

Cover and simmer until rice is done, about one hour.

Add evaporated milk and heat through but do not boil.