Q. Okay, I'm good: I like cauliflower and broccoli. I even like collard greens, when I can find them.
Sorry to keep asking things, but does cooked vs. raw matter?
A. I should emphasize first that vegetables aren't a potent source of omega-3 (n-3).
Whereas a 4 oz. serving of salmon supplies around a gram of n-3, a cup of broccoli or cauliflower supply around 1/100 of a gram of n-3. You'd have to eat 100 cups of broccoli to get the amount of n-3 you get in a measly 4 oz. piece of baked salmon.
(By the way, this is why it's important to establish standards for labeling an animal as "grass fed". They really do need to get the bulk of their calories from greens, not grains, for their flesh to be higher in n-3. I think it's misleading to call an animal "grass fed" if all you do is let it out for sun (and grass) in the afternoon, and supplement it with grain the rest of the time.)
But I wouldn't want to dissuade anyone from a habit of drinking freshly squeezed vegetable juice! It's so health-laden for so many reasons, if not as an n-3 source.
Back to the answer to your question.
The long name for what we're calling omega-3 is omega-3 polyunsaturated fatty acid (n-3 PUFA). Given that n-3 is a fat, it makes sense that we don't find much in cabbages, and that we find lots in oily fish, or in linseed oil or other oils. And given that n-3 is a fat that's very unsaturated (polyunsaturated), it's very susceptible to degradation upon exposure to heat, light, or oxygen. (I've posted a little about degradation of fats at "They're Rancid", "No Smoking, Please", and "I Like Charts".)
We really do need those double bonds in an n-3 PUFA to be intact for it to continue along the path towards becoming an effective inflammation-reducing prostaglandin. Thus, raw is better than cooked. But, for practicality and safety, I don't think a quick steam or a light bake will impact n-3 content significantly. I wouldn't leave a bottle of walnut oil (or any oil) in the sun though. And I'd never fry with a mostly polyunsaturated oil.