PreparationFor Vinaigrette: Combine all ingredients except oil in blender and blend well. Gradually blend in oil. Season to taste with salt and pepper. For Salad: Cut onions lengthwise into 1/2-inch-thick wedges, leaving root ends intact. Place onions in 15x10-inch glass baking dish. Pour 1 cup vinaigrette over onions, coating evenly. Let marinate 3 hours. Chill remaining dressing. (Can be made 1 day ahead. Chill onions.) Prepare barbecue (medium-high-heat) or preheat broiler. Sprinkle onions with salt and pepper. Grill or broil onions until golden. turning occasionally, about 12 minutes. (Can be made 6 hours ahead. Let stand at room temperature.) Place spinach in large bowl. Toss with enough vinaigrette to coat. Season with salt and pepper. Fill radicchio leaves with spinach. Top with grilled onions. Pass remaining dressing separately.
PreparationFor Vinaigrette: Combine all ingredients except oil in blender and blend well. Gradually blend in oil. Season to taste with salt and pepper. For Salad: Cut onions lengthwise into 1/2-inch-thick wedges, leaving root ends intact. Place onions in 15x10-inch glass baking dish. Pour 1 cup vinaigrette over onions, coating evenly. Let marinate 3 hours. Chill remaining dressing. (Can be made 1 day ahead. Chill onions.) Prepare barbecue (medium-high-heat) or preheat broiler. Sprinkle onions with salt and pepper. Grill or broil onions until golden. turning occasionally, about 12 minutes. (Can be made 6 hours ahead. Let stand at room temperature.) Place spinach in large bowl. Toss with enough vinaigrette to coat. Season with salt and pepper. Fill radicchio leaves with spinach. Top with grilled onions. Pass remaining dressing separately.