Step 1Whisk together lemon juice, oil, mustard, herbs de Provence and salt and pepper to taste in small bowl. Slice each chicken breast into 1-inch cubes. Toss chicken in marinade.
Step 2Cut zucchini into 1-inch-thick rounds. Quarter onion and separate pieces.
Step 3Divide chicken and vegetables among 8 skewers, alternating chicken, zucchini and mushrooms. Use 2 mushrooms per skewer, threading from stem through to top to prevent mushroom from breaking in half.
Step 4Grill chicken-vegetable skewers over medium-high heat, basting with remaining marinade, turning often until chicken is cooked and vegetables are tender and beginning to char, about 10 minutes.