Peel and finely chop the onion. Cut the lamb into ¼-inch dice. Put the olive oil and onion in a heavy-bottomed saucepan and place over medium-high heat. Sauté, stirring occasionally, until the onion turns a rich golden color, about 5 minutes.
Chop enough rosemary leaves to measure 1teaspoon and add it to the onion. Stir well, then add the lamb. Sauté until the lamb has lost its raw red color. Add the wine and let it bubble away until it is reduced by about half. While the wine is reducing, coarsely chop the tomatoes. Add them to the pan and season with salt. Adjust the heat so that the tomatoes simmer gently. Cook, uncovered, until the meat is very tender, at least 2 hours. If the liquid in the pan evaporates, add a little water.
While the sauce is cooking, core, seed, and peel the pepper. Cut away any white pith inside the pepper and cut it into narrow strips 1 to 1½ inches long.
Fill a pot for the pasta with at least 6quarts water, place over high heat, and bring to a boil.
Add the pepper to the sauce and cook, covered, until it is quite tender, 15 to 20 minutes.
Add about 2tablespoons salt to the boiling water, put in the fusilli, and stir well. Cook until al dente. When the pasta is done, drain it well, toss with the sauce, and serve at once.
You can make the sauce up to 2 days ahead and refrigerate it. Wait to add the bell pepper until you are ready to cook the pasta.