1. First, in a large resealable plastic bag, combine 1 tablespoon flour and ½ teaspoon salt. Add the beef strips and shake to coat evenly.
2. Next, melt 2 tablespoons butter in a large skillet over medium-high heat. Add the beef and brown for about 3 minutes per side. Add the mushrooms, onion, and garlic; cook and stir until the onion is tender, approximately 3 to 4 minutes. Transfer the beef mixture to a bowl and set aside, leaving the drippings in the skillet.
3. After that, melt the remaining 2 tablespoons butter in the same skillet. Stir in 3 tablespoons flour and tomato paste until well blended. Gradually whisk in the chilled beef stock. Bring the mixture to a boil, stirring constantly, until the sauce thickens.
4. Then, return the beef mixture to the skillet. Reduce the heat to low and simmer for about 5 minutes, allowing the flavors to meld.
5. Meanwhile, cook the whole wheat egg noodles according to package instructions. Drain and set aside.
6. Finally, remove the skillet from heat and stir in the light sour cream until the sauce is smooth and creamy. Serve the beef stroganoff over the cooked noodles.