To make the roast beef:

skip directly to stroganoff recipe if you have leftover roast beef (Pot Roast)

1. Heat a heavy duty skillet over high heat .
2. Pat dry the chuck roast and season evenly with salt, pepper, and rosemary equally on all sides.
3. Preheat oven to 350°F. Use a cast iron Dutch oven if you have one (a heavy baking pan works if covered tightly with heavy duty foil).
4. Place the chuck roast in the hot skillet and sear on all sides, turning as needed with tongs.
5. Transfer to the Dutch oven and add onion, celery, carrots, and garlic, along with enough water to come up halfway on roast.
6. Cover and roast for 2½–3 hours until fork-tender.
7. When roast is done, serve and reserve all liquid and vegetables for the next day.

To make the beef stroganoff: 

1. In a medium stockpot, combine the remaining liquid from the roast with beef base and tomato paste. Add enough water to make 1 quart. As it heats, use a fork to smash the cooked vegetables into small pieces.
2. Bring the mixture to a gentle boil.
3. While the broth heats, melt butter in sauté pan and cook mushrooms over medium-high heat for 5-7 minutes, until browned. Add flour and stir to combine.
4. When the broth reaches a boil, add egg noodles and cook until tender.
5. Stir in mushroom-flour mixture and blend until sauce reaches a gravy-like texture. If it's too thick, add 1/4 cup water or milk until you reach your desired texture.
6. Shred remaining meat with your fingers and add to noodles. Stir in sour cream and season with salt and pepper to taste. Serve warm.