Put the minced onion, mustard, salt, vinegar, garlic and rosemary into a blender and pulse it for about 30 seconds. Use a spatula to scrape down the sides of the blender and purée it again for 10-20 seconds. Scrape the sides down again.
Turn the blender on low and take the removable cap off the lid. Hold your hand over the hole, as it might spit a little.
Pour the olive oil in slowly and put the cap back on.
Turn the blender off and scrape the sides down one more time. Turn the blender back on low, then high for 60 seconds.
Prepare the fish: Rinse the fish under cold water. Now make sure its gills and scales are all removed; your fishmonger is not always so diligent about this task, and no one wants a scale on his plate. Gills can impart a bitter taste to the fish, so they need to go, too.
Make cuts on the sides of the fish: Use a very sharp knife and make several slashes on the sides of the fish, maybe every inch or so. Make the cuts at an angle to the side of the fish, and slice down until you feel the spine. Do not sever the spine, however. These cuts will help the fish cook faster. Rub olive oil all over the fish and set it aside.
Prepare the grill for high, direct heat. Scrape down the grates well and close the lid. Salt the fish well. Now grab a paper towel, a set of tongs, and some cheap vegetable oil. Bring all of this out to the grill.
Fold the paper towel over several times, moisten it with the vegetable oil, and hold it with tongs to wipe down the grill grates.
Lay the fish down on the grill and close the lid.
Let this cook for 5 minutes without touching it.
Open the lid and, using tongs, gently see if you can lift the fish off the grates cleanly. Don’t actually do this, but check for sticky spots. If you have some, get a metal spatula. Use the spatula to dislodge the fish from the sticky spots.
Using tongs in one hand, and the spatula in another, gently flip the fish over. If it sticks, no biggie. It happens sometimes.
Finish grilling the fish: Once the fish has been flipped, let it cook another 3-5 minutes. Again, test for sticky spots with the tongs and spatula. Dislodge them gently and gently lift the fish onto a plate.
If the fish is too long or seems like it might break in half, use two metal spatulas instead of the tongs-and-spatula set-up.
Drizzle the vinaigrette over the fish and serve at once. Goes well with crusty bread and a glass of pilsner beer or white wine.
