Season the chicken breasts on both sides with the onion salt and black pepper.
Heat the olive oil in a large frying pan over medium heat and sear the chicken until golden, about 3 minutes per side, then remove and set aside.
In the same pan, whisk the cornstarch with a few tablespoons of the chicken stock until smooth.
Gradually whisk in the remaining chicken stock, then add the Dijon mustard and honey and stir until combined and bubbling slightly.
Stir in the milk and heavy cream, bring the sauce to a gentle boil, and adjust seasoning with salt and pepper to taste.
Return the chicken to the pan and add the chopped broccoli. Cover, reduce heat to a simmer, and cook until the chicken is cooked through and broccoli is tender, about 10 minutes.
Garnish with chopped fresh parsley if desired and serve.