Peel and finely chop the onion. Put it with the butter in a 12-inch skillet over medium heat and sauté, stirring occasionally, until it turns a rich golden color, about 5 minutes.
Chop the pancetta. Coarsely chop enough of the marjoram leaves to measure 1tablespoon. When the onion is ready, add the pancetta and marjoram to the pan. Cook until the pancetta begins to brown, 2 to 3 minutes. Add the tomatoes and season with salt. Raise the heat to medium-high and cook until the tomatoes are no longer watery, about 15 minutes. Remove from the heat and set aside.
Add about 2tablespoons salt to the boiling water, put in the spaghettoni, and stir until all the strands are submerged. Cook until al dente, then drain well. Toss the pasta with the sauce and serve at once.
You can make the sauce up to 1 day ahead and refrigerate it.