Heat the oil in a large non-stick frying pan, squeeze the sausage out of the casings (discard casings), and cook the sausage over medium-high heat until it's nicely browned, breaking it apart with a turner as it cooks. Don't rush this step; it will take about 10 minutes to get the sausage browned.
When the sausage is nicely browned, push it to one side of the pan, add onions, and cook until they're starting to soften, about 3-4 minutes.
Add the minced garlic, fennel pollen, and Italian herb blend and cook 1 minute more. Transfer sausage/onion mixture to the slow cooker.
Drain garbanzo beans into a colander placed in the sink and rinse well with cold water until no more foam appears.
Let beans drain and then add to slow cooker with the diced tomatoes, crushed tomatoes, and water. (It's okay to use all of one type of canned tomatoes.)
Cook for about 4 hours on high or 8-9 hours on low.
When the beans are quite soft and the flavors are well-combined, add the pesto and cook about 20 minutes more.
Season to taste with salt and fresh-ground black pepper.
Serve hot, with Parmesan cheese to add at the table if desired.
Makes 6-8 servings. The recipe also freezes well.