To make the pesto, combine basil, garlic, pine nuts and Parmesan in the bowl of a food processor; season with salt and pepper, to taste. With the motor running, add olive oil in a slow stream until emulsified; set aside.
In a large bowl, combine chicken, 1/2 cup pesto, Greek yogurt, salt and pepper, to taste.
Serve sandwiches on baguette with arugula, tomatoes, mozzarella and chicken pesto mixture.