Preheat the oven to 400°F. Lightly drizzle the chicken thighs with olive oil and season with salt, pepper, and ground fennel, then add to a large cast iron pan or skillet (working in batches if needed, so as to not overcrowd the pan or skillet), skin side down over medium heat. Allow the fat to render and the chicken skin to become crispy and golden brown, about 10-15 minutes. Using tongs, flip chicken thighs and transfer to a wire rack over a sheet pan and place in the oven. Cook until the internal temperature (at the thickest parts, closest to the bone) reads 160-165 F, about 20 minutes. Reserve the rendered chicken fat in the pan or skillet for the date gravy.
As your chicken cooks, prepare your stuffing: In a large pan over low-medium heat, melt 6 tablespoons butter. Once the butter has melted, add the garlic, leeks, fennel, and dates. Season with salt and freshly ground pepper and cook for about 10-15 minutes, stirring as you go. Increase the heat and add the wine, rosemary, and thyme. Once slightly reduced (about 5 minutes), remove from heat.
In a large mixing bowl, whisk the chicken stock with the eggs. Add the bread and the mixture of cooked garlic, leeks, fennel, and dates to the bowl, and thoroughly combine everything. Transfer to a baking dish and cover with foil. Transfer to the oven and cook for 25 minutes. Remove foil and broil until golden brown and crisp on top (about 5 minutes), stirring once or twice if necessary to ensure any remaining liquid is evenly dispersed and absorbed. Once ready, garnish with orange zest, and, if needed, salt and black pepper.
Prepare your date gravy: Add the shallots and dates to the rendered chicken fat and cook over low-medium heat for 5 minutes, or until fragrant. Increase heat to high and add white wine and chicken stock. Once thickened, lower the heat, add mustard, stir to combine, and taste. If needed, season with salt. Remove from heat and transfer to a gravy boat.
When the chicken is cooked through, remove it from the oven and allow time for it to rest (about 10 minutes). When the stuffing is ready, transfer everything to serving plates and serve alongside warm gravy.