Increase heat to medium-high. Add mushrooms and cook, stirring occasionally, until browned, 6 to 8 minutes. Reduce heat to medium-low and add onions, leeks, and garlic. Cook, stirring occasionally, until light golden brown and tender, 8 to 9 minutes. Add flour and cook, stirring, 1 minute. Gradually add wine, stirring constantly. Add stock, mustard, thyme, and bay leaves; bring to a simmer.