Steps
1. Preheat a grill or grill pan over medium heat.
2. Toss the sliced zucchini, red bell pepper, yellow bell pepper, and red onion with olive oil, salt, and pepper.
3. Grill the vegetables for about 5-6 minutes, or until they are tender and slightly charred.
4. Toast the bread slices.
5. Spread hummus on one side of each bread slice.
6. Layer the grilled vegetables on one bread slice.
7. Top with fresh spinach leaves, sliced tomatoes, and avocado if desired.
8. Cover with the second bread slice, hummus side down.
9. Cut the sandwich in half and serve.