Heat a grill or grill pan over medium heat. Spray the grill or grill pan with cooking spray. In a small bowl. whisk together the olive oil, ground cumin, ground coriander, chili powder and salt. Transfer the vegetables to the grill, cooking on each side for 1 to 2 minutes, taking in mind that the red bell pepper will take longer to cook than the other vegetables. Remove the vegetables and immediately brush them with the olive oil spice mixture.

To assemble the sandwiches, smear a tablespoon of hummus to each bread slice. Arrange a few zucchini slices, red bell pepper, onion and yellow squash to each half of bread; top with opposite slice of bread.

Repeat the sandwich-making process with the remaining bread and vegetables.