Halve an onion through the root and stem ends. Turn one of the onion halves cut side down on the cutting board. Trim off the stem but leave the root end intact. Remove the papery outer skin.
With the root end pointing away from you, make cuts across the face of the onion half, perpendicular to the root. Take care not to cut through the root. The wider you make the cuts, the larger the size of the dice will be. Make thin cuts if you want small pieces of onion. The onion half should remain intact during this step.
Keeping the onion on its flat side, turn it so you’re holding the root end in your left hand if you’re right-handed or your right hand if you’re left-handed. Slice across the onion, perpendicular to the cuts you just made. Again, the wider the cuts are, the larger the dice will be. The layers of onion will fall away in coarse diced pieces.
To make sure the onion pieces cook at the same rate, cut bigger pieces down so they’re all about the same size.