Probably the quickest crop are radishes. I sowed a row of radish seeds four weeks ago and I’m pulling them already. I favour the variety French breakfast because I find the red and white colouration very appealing, and they have a lovely flavour.
To sow radish seed just make a narrow trench not very deep just about twice as deep as the seeds are thick, then sprinkle the seed in the row then lightly cover with soil, water in with a fine rose on your watering can, then wait for roughly a month. You can see when they are ready to harvest as the red shows above the soil and you can then see how big they are and if they’re the right size for you then gently pull from the stem at the base of the leaves and the radish will pop out of the ground.
Radishes also make a good marker crop. When planting parsnips for example. Parsnips are notoriously slow to germinate so I always sow radishes in the same row as parsnips, the radishes are up way before the parsnips and they mark the row for you so you can hoe around them and then as you lift your radishes the parsnips should be through.