It was a treat to see the whole class working so hard on those onions on Wednesday. Bold color, strong darks, bright highlights, believable shadows. And maybe a few of those longitude lines. Do you think you could make a decent onion now, without actually looking at a real one?
This is a good time to practice the skills involved in discovering what needs to be correct in a subject, and what can be carefree.
Find a simple still life object - preferably one with dark darks and light lights. Make a 3-value, monochrome version, rounding up or down so that everything is either white, middle grey, or black.
Assess your study by asking "where do I want more subtlety?"
Write your conclusions directly on the study.
Make another monochrome study, this time using as many values as you feel the subject needs.
Move into color, limiting your palette to no more than 3 colors.
Here are some still life watercolors from various website; all kinds of approaches.