Prepare spicy gremolata: In a medium bowl, add the parsley, thyme, rosemary, sage, garlic, lemon zest, crushed red pepper flakes, Kosher salt and mix to combine. Stir in olive oil. Set aside.
Place the chicken thighs skin-side down on a clean cutting board and, using a sharp boning knife, cut along either side of the bone to loosen it, but still leaving the bone attached to the thigh.
Flip the thighs over and rub the spicy gremolata under the skin of each chicken thigh. Season the opposite side with salt and pepper.
Heat a 12" nonstick skillet over high heat until hot but not smoking.
Nestle chicken in skillet, skin side down, and cook for two minutes.
Reduce heat to medium-high; continue cooking skin side down, occasionally rearranging chicken thighs and rotating pan to evenly distribute heat, until fat renders and skin is crispy and golden brown, about 10-12 minutes.
Turn the thighs over and sear for two minutes, then remove from pan.
Reduce heat to medium; add the sliced onions and allow to cook for about 5 minutes, or until they begin to become translucent and softened.
Add the garlic cloves and cook for another few minutes, until they begin to soften, then stir in the stock
Nestle the chicken thighs, skin-side up, back in the pan and continue cooking for another 6-8 minutes, until the sauce has reduced and thickened slightly and the chicken registers 165°F with a meat thermometer.
Allow to rest for five minutes. Sprinkle gremolata over chicken and serve immediately.