Preheat the grill to high heat.
Slice the peppers into large pieces rather than strips so they do not fall through the grates. If you want slices - use foil or a grill basket to grill the veggies.
Slice the onion into 1/2 inch round pieces.
Use a mandoline to slice the zucchini and yellow squash, or slice thin with a sharp knife lengthwise.
Drizzle or brush the vegetables with olive oil and season with your choice of seasoning blend on both sides.
Place the vegetables on the grill and reduce the heat to medium-high. The high heat will help with the initial char and grill marks, but you don't want to cook them on high heat for too long as they will burn.
Grill the vegetables for 5 minutes on one side, then flip and cook for an additional 5 minutes on the other side. Flip again and cook for another 5 minutes. Grilled vegetables take about 15 minutes to cook to achieve proper texture and maximum flavor.
If using foil or a grill basket, flip the vegetables with a long grill spatula 2 - 3 times over a 15-minute interval to allow proper caramelization on each side.