Set your oven to 180°C fan (200°C regular) or Gas Mark 6. This will ensure it's at the right temperature when you're ready to bake the stuffing.
Melt the butter in a small frying pan over medium heat. Add the finely diced onion and cook gently for 5-10 minutes, stirring occasionally, until the onion is slightly softened but not browned. Remove from the heat, making sure to retain all the butter in the pan.
Remove the skins from the sausages by cutting along their length and turning them inside out. Discard the skins. Place the sausage meat in a large mixing bowl. Add the softened onion along with the butter from Step 2, chopped dried apricots, chopped chestnuts, panko breadcrumbs, minced fresh sage, and a pinch of sea salt. Mix thoroughly until the ingredients are well combined — I find using my hands for this makes blending easier and more even.
Transfer the stuffing mixture from Step 3 into a shallow ovenproof dish. Don’t smooth the surface; leaving it rough creates a nice texture as it bakes. Place in the preheated oven and bake for 45 minutes, or until golden brown and cooked through. For extra flavor, I occasionally scatter a few extra sage leaves on top before baking.