Heat vegetable oil (or butter) in a large skillet over medium heat.
Add chopped onion and cook 4–5 minutes, stirring, until translucent.
Stir in garlic and cook another 30 seconds until fragrant (don’t let it burn).
Add cubed chicken breasts to the skillet.
Season with salt and pepper.
Sear 6–7 minutes, stirring occasionally, until golden brown on the outside and just cooked through. Remove excess liquid if chicken releases too much moisture.
Stir in ketchup and mustard, mixing well so the chicken is evenly coated.
Add tomato sauce and reduce heat slightly.
Simmer uncovered for 8–10 minutes, stirring occasionally, until the sauce thickens and the flavors meld.
Lower heat to medium-low. Stir in heavy cream.
Simmer gently for 2–3 minutes, stirring constantly, until sauce turns silky and slightly pink. Do not boil, or the cream may split.
Spoon a generous portion of rice onto plates.
Ladle the chicken stroganoff over the rice.
Top with crispy potato sticks for crunch., and garnish with fresh parsley if desired.