Prepare the ingredients ahead of time. Finely slice the onion and mushrooms, and mince the garlic. Thoroughly wash the spinach and roughly chop it.
To make the sauce, add the coconut milk, chicken stock, mustard, paprika, garlic powder, and salt to a bowl. Whisk to mix. Set aside.
Add the oil to a large skillet and place it over high heat. Add the chicken thighs and cook for 4-5 minutes per side until golden. Take the chicken off the heat and place it on a plate.
Reduce the heat to medium and add the onions and mushrooms. Sauté for 3-4 minutes or until the onions have softened. Add the garlic and cook for 1 more minute.
Pour the mustard sauce into the skillet. Scrape the bottom of the skillet with a wooden spoon to deglaze. Bring the sauce to a simmer.
Reduce the heat to low and return the chicken to the skillet. Cover and let simmer for 2-3 minutes until the chicken is heated through. Add the spinach and stir.
Let cook for 1 more minute or until the spinach is slightly wilted. Taste and adjust seasoning if needed. Serve immediately.