Dice the chicken breast fillets into small cubes. Heat a large pan over medium-high heat and fry the chicken until it starts to color, about 3 minutes. While the chicken cooks, cut the leeks into rings, the carrots into thin slices, and the mushrooms into quarters.
Add the vegetables to the pan and fry with the chicken over medium-high heat for 5 minutes. Pour in the vegetable stock to deglaze the pan, scraping up any browned bits. Stir in the soy sauce and curry powder, then reduce heat to medium-low and simmer for 8 minutes, or until the chicken reaches 74 °C (165 °F) internally.
Stir in the sour cream and cook for 1 minute. Serve with rice or pasta.