Are fruits always the flesh part of the "fruits"? Is the edible part of the onion a fruit?

In some so-called fruits the actual fruit is not the flesh part. For instance, the flesh part of the strawberry is not the fruit. The fruits are the small hard dots on the surface of the strawberry. Another instance: the flesh part of the cashew is not the fruit. The fruit is the nut.

The edible part of the onion is the stem of the plant and not the fruit.