- Preheat the barbecue to medium-high.
- In a small bowl, mix the yogurt, honey, coriander, chives, and lime zest. Season with pepper to taste. Set aside.
- Remove the mushroom stems and use a spoon to remove the gills. Brush mushrooms with beef broth concentrate.
- Place the mushrooms on an oiled barbecue grill. Place the rolls on the upper grill. Close the barbecue cover and grill 2 to 3 minutes per side. Place cheese on mushrooms and remove from heat.
- Spread the bottom half of each ciabatta roll with yogurt sauce. Place a Boston lettuce leaf on top and then a mushroom cap. Place remaining ciabatta half on top.
Variation: Replace the yogurt sauce with mustard, and add a slice or two of bacon.
Tip: Avoid preparing the yogurt sauce too far ahead of time because the lime zest tends to develop a slightly bitter taste over time.