Add the sliced chicken tenderloins to a large bowl. Season with salt, garlic powder, cornstarch, black pepper, paprika, and milk. Mix until the chicken is fully coated.
Cover the bowl and refrigerate overnight, or for at least a few hours if you are short on time.
Heat the butter and a little olive oil in a large skillet over medium heat. Add the sliced onions and cook until softened.
Add the green peppers and continue cooking until the vegetables are tender and lightly caramelized.
Stir in the sliced mushrooms and cook until they reduce in size. Season lightly with salt and pepper.
Remove the vegetables from the pan and set aside.
In the same pan, add the marinated chicken. Cook over high heat for 8 minutes, stirring often, until the chicken is browned and cooked through.
Reduce the heat to medium. Add the heavy whipping cream, tomato sauce, sour cream, hot sauce, paprika, salt, and Mexican seasoning. Stir until everything is well combined.
Return the cooked onions, peppers, and mushrooms to the pan. Stir everything together.
Let the mixture cook over medium heat for about 5 minutes, allowing it to lightly bubble and thicken.
Taste and adjust the seasoning with more salt, pepper, paprika, hot sauce, or Mexican seasoning as needed.
Serve warm with Mexican rice, refried beans, fresh lettuce, and sour cream.
