Season and Sear the Chicken: Pat the chicken thighs dry with a paper towel and season them generously on both sides with salt and black pepper. Heat the olive oil in a large frying pan over a medium-high heat. Add the chicken thighs to the pan, working in batches if necessary to avoid overcrowding. Sear for 3-4 minutes per side, until they are a deep golden brown. Transfer the seared chicken to the slow cooker pot.

Sauté the Aromatics: Lower the heat under the frying pan to medium. Add the chopped onion to the pan, scraping up any browned bits from the chicken. Cook for 5-7 minutes, until the onion has softened and is becoming translucent. Add the minced garlic, dried thyme, and dried rosemary, and cook for another minute until fragrant.

Deglaze and Combine: Pour about 100ml of the chicken stock into the hot pan to deglaze it, making sure to scrape every last flavourful bit from the bottom. Pour the entire contents of the frying pan over the chicken in the slow cooker.

Slow Cook to Perfection: Add the remaining chicken stock to the slow cooker. Give it a gentle stir to combine everything. Place the lid on and cook on LOW for 5-6 hours or on HIGH for 3-4 hours. The chicken should be incredibly tender and easy to shred.

Prepare the Thickener: About 20 minutes before you’re ready to serve, make the thickening paste. In a small bowl, mash the softened butter and plain flour together with a fork until you have a smooth, uniform paste known as a 'beurre manié'. What works best for me is ensuring the butter is truly at room temperature to avoid any lumps.

Thicken the Gravy: Carefully remove the cooked chicken from the slow cooker and place it on a plate or board. You can shred it now or leave the thighs whole. Turn the slow cooker to the HIGH setting. Whisk the butter and flour paste into the hot liquid in the slow cooker, a little at a time. Continue to whisk until the paste has dissolved and the gravy begins to thicken. This should take about 10-15 minutes.

Season and Serve: Taste the gravy and adjust the seasoning with more salt and pepper if needed. Return the shredded or whole chicken thighs to the gravy to warm through. Serve immediately.