Heat grill to high. Brush 1 tbsp oil over the sausages, and grill on a tray for 20 mins, turning regularly.
Meanwhile, heat the rest of the oil in a saucepan over a low heat. Add most of the spring onion and cook for 5 mins until softened. Add the beans, garlic, and lemon zest and juice, cook for a few mins more, then gently mash with a potato masher to get a thick, creamy consistency. Season and cook for 5 mins more until warmed through. Scatter over the remaining spring onion and serve with the sausages and wilted greens.