Heat 2 tablespoons of the butter and the oil in a large nonstick skillet over medium-high heat. Once the butter is foamy and melted, add the onions and the Brussels sprouts, and stir frequently until nice and caramelized, about 10 minutes. Season the Brussels sprouts with salt and pepper. Add the scallions and chicken broth and steam the vegetables until the liquid is evaporated and the vegetables are tender crisp, about minute or so. Add the lemon zest into the Brussels sprouts to finish.
Meanwhile, heat the remaining tablespoon butter in a small skillet over medium-high heat. Once melted and foamy, stir in the breadcrumbs and cook until toasted and browned, stirring frequently, about 2 minutes. Remove from the heat, season with salt and pepper and stir in the lemon juice. Remove to a small bowl. Sprinkle the breadcrumbs over the Brussels sprouts before serving.