My favourite edible plant in the garden at present is red mustard (Brassica rapa ’Ruby Streak’) also known as mustard ruby streaks. This finely divided salad leaf can grow as tall as 60cm but is usually a bit smaller. The leaves have a sharp lemony flavour with a hot peppery after taste. I graze on it fresh from the garden, add handfuls to salads, chop it and combine with cream cheese on biscuits, or put it into sandwiches. Older leaves are great in stirfries if added at the last minute. The more leaves you harvest the more they will grow. Lime streaks mustard has green leaves and a similar flavour, but I think the red leafed form has the edge.
Plants are easily grown from seed planted for most of the year, or plants can be purchased in most nurseries. Full sun or partial shade is fine and they like a reasonable soil with good drainage (especially at the moment with so much rain). If you dig in some compost before planting plants will grow more rapidly. I also water mine with diluted seasol (seaweed extract) every couple of weeks. Once the weather starts to warm up plants may go to seed, and the leaves will become tougher and more bitter. Collect seed to sow again, or leave them to self sow.