Open-face Grilled Chicken-and-Pepper Sandwiches with Grilled Romaine Wedges

fresh lemon juice, divided

extra virgin olive oil, divided

Kosher salt

Freshly ground black pepper

boneless, skinless chicken breasts

(1/4- to 3/4-inch-thick) artisan whole grain bread slices

romaine lettuce hearts, halved lengthwise with core intact

small, mixed bell peppers (such as red, yellow, and orange), quartered

fresh mozzarella, sliced

pine nuts, toasted if desired