Preheat oven to 400 degrees F. Cut the meat into ¾” cubes, season with salt and pepper.
In a large Dutch oven heat oil, add beef and cook until brown, about 2-3 minutes on each side. Remove from the pan to a small sheet pan and stick in oven.
Reduce the heat to medium. Add butter to melt, then chopped onion. Sprinkle on paprika and sweat onions until soft, but not browned. Add bouillon granules and sliced mushrooms. Sauté for a couple minutes, until soft. Stir in tomato paste and sour cream until everything comes together, finished by the mustard.
Add back beef and any accumulated juices, bring back to a simmer and cook for 1 minute, stirring constantly. Turn off the heat, squeeze in a little lemon juice and stir. Cover and let rest a few minutes.
Serve with noodles/pasta or rice. Sprinkle on the chives.