You'll need to plant the complete produce to grow seeds or more grains.

You can plant carrots, turnips, and onions to make 3 of their respective seeds, and then use those seeds to make more of their respective crops. Flax, barley, jotun puffs, and magecaps produce multiple of their respective crops, which can then be replanted to make even more crops.

like above posts
you start with seeds plant them down to get your food.
now you can plant back this food which grows into the seed plant giving you 3 new seeds when fully grown

so if you start with 3 seeds and go thru this a couple of times you can triple the output with every cicle