Prepare the ingredients:
Clean and slice the mushrooms. Finely chop the onion and garlic. Set aside.

Sauté the mushrooms and aromatics:
In a large skillet or Dutch oven, heat the butter and olive oil over medium heat. Add the chopped onions and sauté for about 5 minutes, until softened and translucent. Add the minced garlic and cook for another 1 minute, until fragrant.

Cook the mushrooms:
Add the sliced mushrooms to the skillet. Stir well and cook for about 8-10 minutes, until the mushrooms release their moisture and begin to brown. Stir occasionally to ensure they cook evenly. The mushrooms should become tender and flavorful.

Add the broth and seasonings:
Sprinkle the flour over the mushrooms and stir to coat. Cook for about 1 minute to eliminate the raw flour taste. Gradually add the vegetable broth, stirring constantly to avoid lumps. Add the Dijon mustard, soy sauce, paprika, thyme, salt, and pepper. Bring to a simmer and cook for 5-7 minutes, until the sauce has thickened slightly.

Stir in the sour cream:
Reduce the heat to low and stir in the sour cream (or dairy-free alternative). Stir gently to combine and cook for another 2-3 minutes, until the sauce is smooth and creamy. Taste the sauce and adjust seasoning as needed with salt, pepper, or additional mustard.

Serve:
Serve the mushroom stroganoff over your choice of egg noodles, rice, or mashed potatoes. Garnish with freshly chopped parsley for a burst of color and added flavor.