Clean and slice the mushrooms into thick pieces.
Heat olive oil in a large skillet over medium heat.
Add chopped onion and cook until translucent, about 3-4 minutes.
Stir in the minced garlic and cook for another minute.
Add the sliced mushrooms and sauté until browned, about 5-7 minutes.
Sprinkle flour over the mushrooms and mix well.
Pour in the vegetable broth, stirring to combine.
Bring to a simmer and let it thicken for about 5 minutes.
Reduce heat, then stir in sour cream and Dijon mustard.
Season with salt and pepper, and cook for an additional 2 minutes.