Here's The Correct Way To Cut Onions For Aesthetically Pleasing Kabobs - Tasting Table
Briefly

Whether you're serving kebobs to a crowd or grilling a small batch for yourself, they need to look inviting. An easy way to lend your skewered meats heaps of visual appeal is to thread chunks of colorful vegetables like zucchini or squash, different varieties of peppers, and, of course, red onions onto them.
To begin, slice your peeled onion through the center, from root to tip. Then take each half, lay it flat on your cutting board, and slice off the top before cutting through the root again so the base of your onion remains intact. This technique ensures they look pretty against your marinated protein and have enough heft to withstand the heat from the grill.
Finally, separate the onion into individual layers that look like curved rhomboids with a substantive perimeter. Their bigger shape means they'll easily withstand being pierced through a skewer without tearing and look attractive when arranged in a uniform pattern along with your protein, veggies, or even tofu if you're making plant-based kebobs.
Threading chunks of onion on your kebob skewers serves several functions. Firstly, it lends meat-based kebobs some much-needed color and vibrancy against the muted color of the protein. Secondly, the onions develop a scrumptious char on their exterior and caramelize in the heat of the grill, imbuing your kebobs with a gentle touch of sweetness.
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