dv said:

Look at this marinade recipe

Https://www.allrecipes.com/recipe/143981/molasses-marinade-for-beef/

The ingredients include 3 rings onion.

To your mind, what does this mean?

It means that I wouldn’t use the recipe. It shouldn’t be unclear.

In any case, onion powder and (presumably) fresh onion?

Nup. No thanks.