Why Salt your Vegetables?
Salting watery vegetables before roasting and grilling is the difference between restaurant-grade dishes and the soggy mess home cooks sometimes find themselves producing. The technique is easy, although it will require patience. The longer you let the salt do its work, the better the result.
Why salt your vegetables?
It's a debated practice, but the reality is that some vegetables simply hold too much water to roast or grill straight from the fridge without becoming mushy. As the vegetables cook, water gets into the fibers, producing sogginess. Because there is too much moisture, the vegetables fail to caramelize, char, or otherwise deliver the desired look, taste, and texture that is the goal of high-heat cooking. The solution is salting, aka disgorging, your vegetables. Sprinkling them with salt and letting them rest draws out the water.
Another reason for doing this is that the salt finds its way into and circulates around the flesh, lending its flavor-enhancing magic throughout the vegetable. If you are restrained and patient, you will barely taste the salt, but you may notice that the vegetable seems to have a more robust flavor.
How to do it
Salting is simple: sprinkle the cut vegetable with salt, then let it rest and drain. You can use a colander placed in a bowl or a cooling rack set in a sheet pan (this is my preferred method - I tell myself a single layer lets it drain better, but I have no proof of that). Before you are ready to cook, blot the vegetables with a paper towel or rinse and then blot (also my preferred method). How much and how long you salt depends on your taste and patience.
I often salt eggplant for 1 hour on the counter while I prep other things with great results. I have also salted vegetables overnight in the fridge (some chefs do this as standard practice). I have found that you need to salt generously and then rinse for shorter timeframes, but you can lightly sprinkle, as you might just before roasting, for longer timeframes.
When it comes to flavor, my preference is to salt for a whole day. I did this with zucchini boats, and the result was a still-firm and very flavorful stuffed zucchini even after baking and then broiling. That said, I recognize not everyone has that kind of time, patience, or penchant for meal planning. I recommend at least an hour but as much time as you can. Regardless of how long you do it, it's a step worth the wait.