1. Preheat the oven to 350°. On a work surface, using the flat side of a large knife, mash the garlic with 1 teaspoon of salt until a paste forms. Spread open the pork roast on the work surface and rub the inside with the garlic paste. Season the pork with pepper. Roll up the pork to form a neat roast and tie at 1-inch intervals. Season the outside of the roast with salt and pepper, then rub with the brown sugar.
2. In a large roasting pan, combine the sliced onions with the tomatoes and their juices, the ketchup, water and Tabasco. Set the pork roast on top of the sauce, cover with aluminum foil and bake for 2 hours. Increase the oven temperature to 450°, remove the foil and bake for 45 minutes longer, basting occasionally until the roast is nicely glazed. Transfer the roast to a carving board, cover loosely with foil and let rest for 10 minutes.
3. Meanwhile, pour the sauce into a large saucepan and simmer over moderately high heat, stirring occasionally, until thickened, about 12 minutes. Season with salt.
4. Untie the roast and slice it 1 inch thick. Serve with the sauce.
Make Ahead: The baked pork roast can be refrigerated overnight. Slice the roast and bring it to room temperature, then cover with foil and reheat in the tomato sauce.
Serve With: Mashed potatoes, rice or pasta.
Wine Recommendation: A spicy Zinfandel with plenty of sweet berry fruit matches the heat from the Tabasco and blends with the sweetness from the onions and ketchup in the sauce. Try the inexpensive 2000 Cartlidge & Brown California or the 2000 Rancho Zabaco Dry Creek Valley Chiotti Vineyard.