Heat a 4qt dutch oven over medium high heat, add oil, when hot add onion, carrot and celery, season with salt and pepper and saute, stirring occasionally for approx 3 min. Then add mushrooms and continue to saute, after 5 min., season with salt again. When onions are translucent, add butter, thyme and garlic, continuing to stir. When butter is melted and beginning to boil, add flour stirring well and continue to stir, flour mixture will be thick and like a paste, cook flour mixture for 1-2 minutes so the flour has time to take on other flavors...then stir in broth and warmed milk slowly, stirring constantly. Continue stirring over medium high heat until it comes to boil, season with salt and pepper again, boil 1 minute remove from heat and add cream and cheese, if desired.