To make the rouille, in a small saucepan, bring the wine to a simmer and reduce it by half. Set aside. In a food processor, combine the roasted pepper, Tabasco, potato, garlic, salt and pepper and process until almost smooth. Do not over process or the potato will become gummy. Add the olive oil and vinegar in a thin stream while the machine is running, then transfer the mixture to a bowl. Just before serving, heat the reduced wine slightly and beat it into the rouille, drop by drop, to loosen it. Bring a medium saucepan of lightly salted water to a boil. Add the whole potatoes and simmer for 10 minutes.
Drain well, cut in half, and set aside. Preheat the oven to 350 degrees. Heat a heavy 12 inch skillet, preferably cast iron, over medium high heat and add half the olive oil. Sear the onion slices until almost charred, pressing them down with the back of a spatula and turning to the other side when blackened.
Separate the onions into rings and add the garlic and rosemary. Cook, stirring frequently, for 1 minute. Transfer the vegetables to a plate and wipe the pan with a paper towel. Add the remaining olive oil and, again over medium high heat, lightly season and then sear the monkfish tails, turning with tongs to be sure all sides brown evenly. Return the onion mixture to the skillet around the fish and add the potatoes. Season generously with salt and pepper. The fish should not be crowded or it will steam rather than roast. Transfer the skillet to the oven and roast for about 15 minutes, or until the fish is cooked through and the potatoes are tender. Serve the rouille on the side.
Bell peppers can be roasted over a gas flame or on a tray under the broiler. Keep turning so the skin is evenly charred, without burning and drying out the flesh. Transfer the charred peppers to a plastic bag, tie the top closed and let steam until cool to the touch, about 15 minutes. (If you are rushed, you can place the bag in a bowl of iced water to speed things up.) The best way to peel is just to pull off the charred skin by hand and then dip the peppers briefly in water to remove any blackened bits. Do not peel the pepper under running water since that will wash away flavorful juices.