Chop the chicken breast into 2-inch pieces and season with 1/2 teaspoon salt, pepper, and 1 teaspoon lemon juice. Leave it to marinate until you get the sauce ready.
Roughly chop the onions, garlic, yellow pepper, and chili pepper. Place them in a food processor or blender jar along with Dijon mustard, lemon zest and rest of the salt and lemon juice. Blend until you get a smooth, bright yellow paste.
Heat oil in a frying pan and add the marinated chicken pieces. Cook covered on medium heat, stirring occasionally, for 5 to 10 minutes until all the chicken pieces turn white.
Remove the chicken pieces and melt butter in the same pan. Stir in the sauce and simmer over low heat for 15 minutes or until the sauce begins to thicken. Add the chicken pieces and yogurt and cook for another 10 minutes on low heat, until the sauce reaches the desired consistency. Garnish with cilantro and serve.
Do not skip the yogurt. It gives a creamy consistency to the sauce you don't want to miss.
If you have the time and energy, you can roast the bell pepper before blending. It gives an interesting twist to the flavor of the sauce.