- 1/2 cup water
 - 1/4 cup white wine vinegar
 - 3 tablespoons tahini (sesame seedpaste)
 - 2 tablespoons coarse-grained mustard
 - 1 teaspoon honey
 - 1 small garlic clove, minced
 - 3/4 vegetable oil
 - 2 large red onions
 - 12 cups (packed) baby spinach,trimmed
 - 10 large radicchio leaves
 
- Combine all ingredients except oil in blender and blend well. Gradually blend in oil. Season to taste with salt and pepper.
 - Cut onions lengthwise into 1/2-inch-thick wedges, leaving root ends intact. Place onions in 15×10-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.