洋葱既可作可数名词也可作不可数名词。

onion

n.洋葱;葱头

复数： onions

例句：

When the oil is hot, add the sliced onion

油热了后，放入切好的洋葱。

Spread the onion slices on the bottom of the dish.

把洋葱片铺在盘底。

Peel the onion and cut it in half lengthwise.

剥掉洋葱皮并把它纵向切成两半。

It is made with fresh minced meat, cooked with onion and a rich tomato sauce.

这道菜用鲜肉沫加洋葱和浓郁的番茄酱做成。