Preparation Whisk first 6 ingredients in small bowl. Season with salt and pepper. (Sauce can be made 2 days ahead. Cover and chill.) Prepare barbecue (medium heat). Form ground turkey into four 1/2-inch-thick patties. Sprinkle with salt and pepper. Grill burgers until cooked through, about 5 minutes per side. Grill buns until just toasted. Spread 1 tablespoon sauce on bottom half of each bun. Top each with 1 burger, then 1 more tablespoon sauce. Top each with 1 onion slice, 2 tomato slices, then 1/2 bunch arugula. Cover with bun tops. Serve, passing remaining chipotle tartar sauce separately. *Chipotle chilies canned in a spicy tomato sauce, sometimes called adobo, are available at Latin American markets, specialty foods stores and some supermarkets.
Preparation Whisk first 6 ingredients in small bowl. Season with salt and pepper. (Sauce can be made 2 days ahead. Cover and chill.) Prepare barbecue (medium heat). Form ground turkey into four 1/2-inch-thick patties. Sprinkle with salt and pepper. Grill burgers until cooked through, about 5 minutes per side. Grill buns until just toasted. Spread 1 tablespoon sauce on bottom half of each bun. Top each with 1 burger, then 1 more tablespoon sauce. Top each with 1 onion slice, 2 tomato slices, then 1/2 bunch arugula. Cover with bun tops. Serve, passing remaining chipotle tartar sauce separately. *Chipotle chilies canned in a spicy tomato sauce, sometimes called adobo, are available at Latin American markets, specialty foods stores and some supermarkets.