Turn on the slow cooker and use the sauté function, setting the temperature at 275 degrees. Sauté the onion for about 6 minutes, and then add the fennel and celery. Sauté another 3-4 minutes before adding the fennel seeds and garlic.
Add the diced tomatoes, tomato paste, red wine, bay leaf, and oregano and cook on low for 5-6 hours.
Put your pot of water on to boil for your linguine, and add about 2 tablespoons of salt once you are at a rolling boil. Add the linguine and stir occasionally to make sure it doesn't stick.
Taste your sauce, and add a bit of salt and pepper.
Cut each of the sausages in 5-6 pieces and then buzz in the food processor, until you have coarse pieces. Add the sausage pieces to the sauce and cook on low while the linguine is cooking.
Before draining the linguine, reserve a cup of starchy pasta water.
Drain the linguine and add it to the sauce, along with about 1/2 cup of the starchy pasta water and toss to combine (adding a little more pasta water only if necessary to get a good consistency and perhaps a teaspoon or two of extra virgin olive oil.)
Serve with optional Parmigiano-Reggiano.