Heat butter and oil in a 6-quart pot over medium-high heat until butter starts to sizzle, then reduce heat to medium. Add pancetta, and cook 3 minutes or until pancetta begins to brown and fat has rendered. Add onion, carrot, celery, mushrooms and salt and cook 10 minutes, stirring often, until just beginning to brown around edges. Add garlic and cook 2 minutes.
Pour in wine and cook 6 minutes, stirring to scrape up browned bits from bottom of pot, or until liquid has evaporated. Add 3/4 cup milk and cook 3 minutes or until reduced slightly (don't worry if it appears slightly curdled, it will smooth out again). Add crushed and diced tomatoes, stock, thyme and bay leaves and bring to a boil over high heat.
Tip: To make this Keto friendly, serve over zucchini noodles.