Add the chopped onion to a skillet with the olive oil.
Saute until it starts to look translucent, about 5 minutes. Add in the garlic and saute an additional 2-3 minutes.
Add in the sliced mushrooms and saute, covered but stirring occasionally, for about 5-7 minutes or until the mushrooms start to look soft and shrink in size.
Transfer to a blender with all remaining ingredients and blend until smooth. It'll probably look thin at this point, but it will thicken up in a second!
Give it a quick taste and adjust flavors as desired. I like a touch extra rosemary, thyme and black pepper.
Transfer to a pot on the stove and let simmer over low heat to thicken. If it doesn't thicken to your desired consistency after about 10 minutes, just whisk in an additional tablespoon of flour at a time. On the other hand, if it at any point gets too thick, just add 1/4 cup more broth until it reaches your desired consistency. Keep in mind it will thicken in the fridge, so if you're prepping this in advance, avoid adding too much flour.
Serve warm with all of your Thanksgiving favorites!