To best see the rather translucent cells of the common onion needs staining, but if you use the red onions, and use one of the outer layers, then this is not necessary. A temporary mount can easily be made by placing a small piece of tissue on a slide with a drop of water and covering with a slip. Normal epidermal cells look like those in the photomicrograph below. These cells are what is called 'turgid' that is they are pretty full of water and hence there is enough pressure against the rigid cell wall for the cells to keep their brick-like shape. Notice that the red cytoplasm is throughout the whole cell and the only non-red part is between the cell membrane and the cell wall.
Water is drawn out of the cells, shrinking the vacuole and the cytoplasm.
This is seen in the photomicrograph below, the red areas being much smaller. This process is called 'plasmolysis' and the cells are now said to be 'flaccid'. Such cells lend little support to the structure of the pant, indeed this is the very reason why plants wilt if put into salty water. Both photomicrographs by Mike Samworth. If any reader wishes to ask about any of the above, or to comment, please do get in touch by contacting me Mike Samworth .