Season beef tenderloin strips with salt and black pepper. Heat 1/2 tablespoon of oil in a non-stick skillet over medium-high heat.
Working in batches if needed, sear the beef for 1-2 minutes per side, just until browned. Do not overcook. Transfer to a plate.
Add 1/2 tablespoon butter and 1/2 tablespoon oil to the pan. Add the mushrooms and cook, stirring occasionally, until browned and their liquid evaporates, about 4–5 minutes. Transfer to the plate with the beef.
Reduce the heat to medium. Add the remaining 1/2 tablespoon of oil and 1/2 tablespoon of butter. Add the onion and cook for 4–5 minutes or until softened.
Stir in the flour and cook for 1 minute, stirring constantly.
Gradually add the wine, whisking to smooth out lumps. Bring to a boil and let the wine reduce by 1/4th.
Add the tomato paste and give it a quick stir, then add both broths slowly. Bring to a boil and cook for 3-4 minutes, or until the sauce thickens. Reduce the heat to low.
Place the sour cream in a small bowl. Gradually whisk in 1/2 cup of the sauce to temper it. Repeat with another 1/2 cup, then stir the mixture back into the pan.
Add the beef and mushrooms to the sauce along with any accumulated juices on the plate. Add brown sugar, if desired, to balance flavors. Warm through gently, but do not boil.
Stir in half the chopped parsley. Adjust seasoning with salt and black pepper.
Serve over egg noodles. Garnish with remaining chopped parsley.