Combine the mayonnaise, sour cream, parsley, chives, tarragon, anchovy, and garlic in a food processor; pulse about 10 times to blend. The mixture should be a pale green. Transfer to a small bowl; set aside.
Prepare a medium-hot fire in a charcoal grill with a cover, or preheat a gas grill to medium-high.
To make the patties, combine the buffalo, beef, horseradish, onion and salt in a large bowl. Handling as little as possible, shape into 6 patties.
Brush the grill rack with vegetable oil. Place the patties on the rack, cover, and cook, turning once, 5 to 7 minutes per side for medium, or longer if one desires well done. Top the patties with the cheese during the last 2 minutes of grilling. Place the buns, cut side down, on the outer edges of the grill rack during the final minute of grilling to toast to a light golden brown.
To assemble the burgers, spread the bottom halves of the toasted buns with the mustard, layer with the salad greens and the cheese-topped patties, spread with the herbed mayo and sour cream mixture, and cover with the bun tops.
Makes 6 burgers
Comments: With regard to the hamburger buns, I teamed this recipe with cracked-wheat buns, which I bought at Whole Foods; whole-wheat buns are also fine. I use the best-quality in-store bakery/artisan buns available.
Rick Rohr, Darien, Connecticut