Heat the grill to medium heat (about 350-400 degrees).
Cut the zucchini and yellow squash in half lengthwise.
Starting with a whole onion, cut the onion in half root to tip. Keeping the root end intact will help hold it together on the grill.
Place the cut zucchini, yellow squash and onion on a baking sheet. Brush all the cut sides with melted ghee and sprinkle with salt and pepper.
When the grill is up to temperature, place the cut side of the vegetables face down on the grill. The whole bell pepper can sit directly on the grill as well.
Close the lid and grill for about 10 minutes. Turn the bell pepper every couple minutes to get a char on all sides. Check the vegetables periodically to make sure they don't burn. You just want some good grill marks on the cut sides.
When the vegetables are done, remove them from the grill and let sit for 5 minutes.
When cool enough to touch, carefully cut the vegetables and transfer to a medium mixing bowl. Slice the yellow squash and zucchini 1/4" thick. Dice the red bell pepper, and roughly chop the red onion. Please be careful cutting the onion as it will be a bit slippery!
Add the pesto to the mixing bowl and carefully toss the vegetables to combine.
Top with a bit of chopped parsley to garnish.
Serve warm or at room temperature and enjoy!