In large sauté pan, warm butter and olive oil over medium heat. Add chicken strips and sauté until browned but not cooked through, about 5 minutes. Remove chicken from pan and place into bowl to reserve while making sauce.
In same pan, add shallots; sauté for 1 minute. Add mushrooms, salt and pepper and sauté until mushrooms have released all of their liquid, about 7 minutes. Add Dijon mustard, paprika and tomato paste. Stir until combined. Add chicken stock, cover and bring to a boil. Cook to reduce liquid by half, about 12 minutes.
To sauce, add sour cream and stir to combine. Cook over medium heat until sauce thickens slightly. Add reserved chicken and stir to combine. Warm over low heat for another 5 – 6 minutes, or until chicken is cooked through.
Garnish stroganoff over cooked noodles.