Heat the grill to 500 F or higher
Slice Zucchini lengthwise into 3/8" - 1/2" thick pieces
Slice onions against the direction of the core into 1/2" thick rings
Cut off the outside of the bell peppers into large pieces. Approximately 4-5 pieces per pepper
Lightly coat the veggies with 2 tablespoons of olive oil
Season all veggies with Salt & Pepper
Mix equal parts of olive oil and balsamic. Whisk together so the mixture is an emulsion - so you can no longer tell the difference between the oil & balsamic.
Place onions on the grill. Brush on balsamic. Turn after 5 mins and brush again. Grill for 5 more mins.
Place remaining vegetables on the grill. Brush with balsamic. Grill & flip all vegetables every 3 minutes until tender, but not floppy. Zucchini & peppers should only need one flip.