Leaves and Stem
Mignonette is a hairless, sprawling, 'floppy' biennial. Generally, it is about 12 inches tall but can be significantly taller / bigger. There may be a basal rosette of leaves.
The leaves are deeply cut and have a pinnate appearance; the margin is wavy - see adjacent image. The stem has a number of ridges - see image below.
Flowers and Fruits
The upper pair of petals have lobes, and there are numerous stamens (between 12 and 20).