Holding the vegetable or fruit firmly with your fingertips tucked under, cut uniform slices lengthwise with a sharp chef’s knife. Stack two or three slices and cut them lengthwise to the same thickness as before; you now have square sticks. Holding the pile firmly, cut crosswise to form cubes.
To dice an onion finely, cut it in half from stem to root; place the cut side against the board. Make thin slices from root to stem, inserting the tip of the knife first, then pushing downward; do not cut all the way through the root. Next, make thin slices horizontally, as far as the root. Finally, cut across the onion to form tiny cubes.