Grilled Chicken and Romaine Salad with Croutons and Lemon-Herb Dressing

For the lemon-herb dressing:

mayonnaise

chopped basil leaves

scallion, thinly sliced

Juice and zest of 1 lemon

large chicken thighs, boneless, skinless

kosher salt and freshly ground pepper

4-inch piece of baguette, cut into rough 1-inch cubes

medium red onion, cut in 6 to 8 wedges

medium hearts of romaine, halved lengthwise

canola oil