Heat grill to high.
Slice zucchini and onions lengthwise.
Drizzle with olive oil & sprinkle with salt and pepper.
Turn grill down to medium high heat.
Place the veggies on grill and cook on each side - about 3 to 5 minutes.
Remove veggies to platter, drizzle with additional olive oil and fresh lemon juice or balsamic vinegar.
Season with salt and pepper.