How to Make It
In a medium saucepan, heat the oil until shimmering. Add the onion and cook over moderate heat, stirring occasionally, until softened and golden, about 7 minutes. Add the stout, cream and bread and bring just to a boil. Simmer over moderately low heat, stirring occasionally, until the bread is very soft, about 10 minutes. Transfer the sauce to a blender and puree until smooth. Season the sauce with salt and pepper and keep warm over very low heat.
In a large, deep skillet, heat the oil until shimmering. Add the brussels sprouts and cipollinis and cook over moderate heat, stirring, until just starting to brown, 5 minutes. Add the spring onions and 1 cup of water and cook, stirring, until the vegetables are crisp-tender and the water is absorbed, 5 minutes. Season with salt and pepper and keep warm.
Light a grill. Brush the steak with oil and season with salt and pepper. Grill over moderately high heat, turning once, until charred outside and medium-rare within, 5 minutes per side. Transfer to a carving board and let rest for 5 minutes. Slice the steak against the grain and serve with the vegetables and stout sauce.