Into a saucepan over medium-high heat, add Unsalted Butter (5 tablespoon). Once melted and bubbling, add Onion (1). Cook for 2-3 minutes.
Add Mushroom (0.5 pound). Cook for 3-5 minutes.
Add Kosher Salt (1 1/2 teaspoon) and Garlic (1 clove). Cook for 30 seconds, stirring to combine.
Sprinkle All-Purpose Flour (4 tablespoon) over evenly. Stir to combine. Cook for 1 minute.
Add Vegetable Stock (2 cup), Liquid Smoke (1 teaspoon), and Coarse Black Pepper (2 teaspoon). Mix well, then increase heat to high and start to bring the mixture to a boil.
As mixture thickens, add Sour Cream (3/4 cup). Stir to combine. Bring to a full boil and let cook for 6 minutes.
Reduce heat to medium. Add Egg Noodles (12 ounce). Toss together.
Garnish with Scallion (to taste) and Grated Parmesan Cheese (to taste) before serving.