In a large pan, melt 3 tablespoons of butter over medium heat. Add cubed sirloin, season with salt and pepper and cook until completely browned (depending on your pan size, you might even need to do this in batches). Transfer to a plate and set aside.
In the same pan, cook shallots until soft and tender, about 1 or 2 minutes. Remove from pan and set on top of beef.
Still using the same pan, melt 2 tablespoons of butter and cook sliced mushrooms, stirring occasionally, until browned and softened. Season with tarragon and ground nutmeg.
Reduce heat to low and stir in soured cream. If needed, add 1 or 2 tablespoons of water or stock to thin the sauce. Add beef and shallots and season with salt and pepper if needed. Warm through, being careful not to let the sauce boil or simmer as this will cause the soured cream to curdle.