Red onion and Spanish onion

Red onion noun - Flat mild onion having purplish tunics; used as garnish on hamburgers and salads.

Spanish onion and red onion are semantically related. In some cases you can use "Spanish onion" instead a noun phrase "Red onion".

Spanish onion noun - Large mild and succulent onion; often eaten raw.

Red onion and Spanish onion are semantically related. Sometimes you can replace phrase "Red onion" with "Spanish onion", this phrases are similar.