In a small bowl, combine the softened butter, crushed garlic, thyme, rosemary, lemon zest and a generous pinch of salt and pepper until well combined.
Pat the chicken dry with paper towel. Carefully loosen the skin over the breasts and thighs using your fingers, taking care not to tear it. Spread most of the butter mixture evenly underneath the skin, then rub any remaining butter over the outside of the chicken.
Pierce the whole lemon several times with the tip of a knife and place it inside the cavity of the chicken.
Arrange the onion wedges over the base of the slow cooker, then place the chicken breast-side up on top. There's no need to add any liquid—the chicken will release plenty of its own juices as it cooks.
Cook on LOW for 5-6 hours (recommended) or HIGH for 3 hours, or until the chicken is cooked through and the meat is falling away from the bone. We prefer cooking on low, as it produces more cooking juices for the gravy.
Carefully lift the chicken from the slow cooker (it will be very tender) and transfer it to a lined baking tray. Place under a hot grill, or into a 220°C oven, for 8–12 minutes, or until the skin is golden, crisp and bubbling.
Strain the cooking juices into a saucepan, discarding the onion and any large pieces of herbs if desired. Skim off any excess fat if you prefer a lighter gravy. Squeeze the softened lemon from inside the chicken into the saucepan.
In a small bowl, whisk together the flour and water until completely smooth. Bring the cooking juices to a gentle simmer, then slowly whisk in the flour slurry. Continue cooking for 2–3 minutes, stirring constantly, until the gravy has thickened. Taste and season with salt and pepper if needed.
Carve the chicken and serve with the warm lemon gravy and your favourite vegetables.
