Prepare the chicken by removing any fat and sprinkling it with salt and pepper.
Heat a large skillet over medium-high heat. Once hot, add olive oil. When the oil is shimmering, add the chicken thighs skin side down. Let cook until golden brown on each side, about 4 minutes per side. Remove from pan and transfer to the slow cooker.
To the same pan, add onion, garlic, and saute until vegetables are softened and fragrant. Transfer this mixture to the slow cooker as well.
In a small bowl, whisk together chicken broth, honey, dijon mustard, red pepper flakes, and soy sauce. Pour this mixture over the chicken and vegetables in the slow cooker.
Cook on low for 4-6 hours or high for 2-3 hours, until chicken is cooked through and tender. Once done, carefully remove it from the slow cooker (it will be very tender) and sprinkle it with fresh parsley. Serve it with rice or whatever you like.