Wash chicken breast fillets, pat dry, and cut into thin slices. Toss with flour to coat.
Peel onion and finely dice it. Clean, core, and finely dice the red bell pepper.
Cook rice in salted water according to package instructions.
Heat oil in a pan over medium-high heat. Fry the floured chicken slices for 3-4 minutes total until lightly browned on both sides and cooked through (internal temperature 74°C / 165°F). Add the diced onion and fry for 1-2 minutes more.
Stir in the mustard and season with salt and pepper. Pour in the vegetable stock and bring to a simmer over medium heat. Simmer for 5 minutes.
Fold in the crème fraîche and half of the diced red pepper. Taste and adjust seasoning with salt and pepper if needed.
Serve the chicken mixture over the cooked rice. Garnish with the remaining diced red pepper.