Combine the stock and halved garlic head in a big saucepan and simmer for about 15 minutes to give the stock a nice, garlicky taste; strain out the garlic. Keep warm.
Bring a pot of salted water to boil for the rigatoni.
Pour 1/4 cup olive oil in a large saucepan. Add the sage, rosemary and thyme and warm the oil over medium heat to infuse it with the flavor of the herbs, 3 to 4 minutes. Add the sausage and cook, breaking up the sausage with the side of a big spoon until well browned. Chop the carrots, celery, and onion in a food processor. Add to the saucepan and cook for 3 to 4 minutes, until the vegetables are softened but not browned.
To the pan with the sausage stir in the crushed tomatoes, bay leaf, cannelloni beans, and chicken stock. Bring to a simmer and cook for 15 minutes stirring occasionally.
Cook the rigatoni in the boiling water for 6 minutes; it should be slightly underdone. Drain and stir into the simmering soup. Add the parsley, and salt and coarsely ground black pepper, to taste. Discard the bay leaf and herb sprigs.
To serve, preheat the broiler. Put the baguette slices in a single layer on a baking sheet. Sprinkle with the Parmigiano and broil until the cheese is bubbly and golden brown, about 3 minutes. Ladle the soup into bowls and float a couple of the baguette slices on top.