Dice bacon and heat a large skillet over medium heat. Place bacon in the skillet to cook, stirring occasionally, until it is just browned. Remove and place on a paper towel lined plate.
While the bacon is cooking, make the mustard sauce by combining all of the mustard sauce ingredients in a bowl or large measuring cup. Whisk to mix.
Chop, onion, mushrooms and kale. Measure out remaining ingredients.
When the bacon is finished, reserve bacon fat in the pan and keep the skillet on medium heat. Place the chicken in the skillet to cook. Cook for 4-5 minutes, without disturbing. Flip it and cook for another 4-5 minutes. If the chicken is just slightly undercooked, that's ok. It will finish cooking with the sauce. Remove and place on a plate.
Place onion in the skillet and cook, stirring occasionally, for 3-4 minutes, until the onion starts to soften.
Add the mushrooms and cook, stirring occasionally, for another 2-3 minutes.
Give the mustard sauce a quick whisk to mix and pour it into the skillet. Scrape to deglaze with a wooden spoon. Bring the sauce to a quick simmer.
Add the chicken and bacon back into the skillet. Cover, turn down heat and simmer for 4-5 minutes. The sauce should thicken a little. Ensure that the chicken is cooked through.
Remove cover and add in the kale. Stir to mix for 2-3 minutes, until the kale is wilted.
Serve chicken topped with chopped parsley if desired. Spoon some of the sauce over the chicken for serving.
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