Preparation Mix sour cream, yogurt, vinegar, grated garlic, honey, and 5 Tbsp. oil in a small bowl; season dressing with salt. Heat remaining 3 Tbsp. oil in a medium skillet over medium. Add crushed garlic and scallions and cook, stirring often, until golden brown around the edges, about 4 minutes. Add panko and season with salt. Cook, stirring often, until golden brown, about 3 minutes. Finely grate lemon zest directly into pan and toss a few times to incorporate. Transfer breadcrumbs to paper towels to drain; let cool. Taste and season with more salt if needed. Place radicchio in a large bowl. Drizzle with dressing; toss gently to coat. Season with salt and scatter breadcrumbs over. Do Ahead Dressing and breadcrumbs can be made 2 days ahead. Cover and chill dressing. Store breadcrumbs airtight at room temperature.