Onions Grow On Little Mesh Bags
But of course the example gains its force from the fact that it's actually too extreme to be true. We know steaks actually come from cows, not plastic packages (and hence we're led to agree with the author about the ridiculousness of those other Average Ignorant Consumers). What's more, we all have a general idea of what a cow is like. Certainly we have a great deal of ignorance about how livestock is raised and processed, but we know what animal is involved for anything but the dodgiest sausage.
Vegetables, on the other hand, are shrouded in even more mystery. How many people know what kind of plant a potato grows on, much less what that plant looks like? And even our vague sense of tropical versus non-tropical crops is unable to provide a modicum of information about the growing habits of many vegetables. Our knowledge is better for some, either because they're iconic (e.g. corn) or because they're often sold whole (e.g. carrots). But an average person in the US would have difficulty describing the plants that much of the produce section came from, whereas errors in the meat department would be limited to the exact shape of a few kinds of fish.
There are a few reasons that meat might seem like a more compelling illustration of our ignorance of where our food comes from. One is the cross-fertilization of concern about being out of touch with our food system and the "visceral argument" for vegetarianism. The most terrible aspects of meat production are more immediate (slaughtering a living creature and seas of manure, versus migrant farmworkers being underpaid and monocultures destabilizing ecosystems), so ignorance and not-thinking-about-it seem to play more functional psychological roles with respect to meat*. Second, at a certain scale the difference in appearance between meat and its source is greater -- though we may not know what a whole potato plant looks like, the potato part looks like a potato, whereas there's no part of a living cow that you can see that looks like a steak. So it's easy to forget where our ignorance really lies.
*Despite its intuitiveness to some people, I don't think the visceral argument carries much weight. People are quite good at adapting to and accepting cruelty that isn't directed at them.