Preheat oven to 400°F

To prepare packets, start with four 20- to 24-inch-long pieces of parchment paper or foil. Fold in half crosswise. With the parchment or foil folded, draw half a heart shape on one side as you would if you were making a Valentine. Use scissors to cut out the heart shape. Open up the heart.

Place a chicken breast on one side of each open heart fairly close to the crease and leaving at least a 1-inch boarder around the edges for folding and spread with mustard.

Toss zucchini, peppers, shallots, tomatoes, oil, thyme, salt and pepper in a large bowl. Place about 1 1/4 cups of the vegetable mixture on top of each piece of chicken.

Close the packet to cover the ingredients. Starting at the top, seal the packet by folding the edges together in a series of small, tight folds. Twist the tip of the packet and tuck it underneath to help keep the packet closed. Place the packets on a large rimmed baking sheet (packets may overlap slightly). Bake about 25 minutes.