Cut the onion, garlic and pepper into cubes. If you have yellow and red peppers, even better, I didn't use them.
In a pan heat a spoon of oil and put the vegetables already cut, you can put them all together.
Saute until they are soft.
Remove from pan and set aside.
In the same pan, add a tablespoon of oil and sauté the chicken until golden and dry.
While the chicken is sautéing, add salt to taste, I used the Xodó seasoning from @xeroespeciarias
When it's golden add the ingredients you had sautéed before, stir and turn off the heat.
In another pan put a glass of water with a spoon of corn starch, the soy sauce and the mustard, stir until it thickens.
Turn off the heat and add to the chicken and vegetables, the chess chicken is ready.
It's easy and fast!