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"La cebolla" is a form of "cebolla", a noun which is often translated as "onion". "Los guisantes" is a form of "guisante", a noun which is often translated as "pea". Learn more about the difference between "la cebolla" and "los guisantes" below.
la cebolla(
seh
-
boh
-
yah
)A feminine noun is almost always used with feminine articles and adjectives (e.g., la mujer bonita, la luna llena).
1. (vegetable)
el guisante(
gee
-
sahn
-
teh
)A masculine noun is used with masculine articles and adjectives (e.g., el hombre guapo, el sol amarillo).
a. pea 
El chef preparó una ensalada de gambas con patatas y guisantes.The chef prepared a shrimp salad with potatoes and peas.