The “AnchoisFert” solid residue comprised of milled anchovy leftovers after fish oil extraction with biobased limonene is a powerful organic fertilizer. Employed to promote the growth of Tropea’s red onion (Allium cepa), the fertilizer turns out to largely superior to commonly used organic (manure) and chemical (NPK) fertilizers. Rich in proteins, organic carbon, flavonoids, magnesium, potassium, phosphate and sulphate, and devoid of antibiotics and antibiotic resistance genes, the new organic fertilizer can replace both conventional organic and inorganic fertilizers. The discovery closes the fishing material cycle for one of the most fished species across the seas opening the route to a new class of organic fertilizers of exceptional performance derived from abundant biowaste via a low-cost, environmentally-friendly circular economy process.