1. Bring a large saucepan of water to a boil for the pasta.
2. Slice off the fennel stalks and reserve any fronds for an optional garnish (roughly chop them). With the fennel bulb, if the outer layer is very thick, remove it. Slice the bulb in half, lengthwise. Use a mandoline or sharp knife to thinly slice the fennel crosswise, as thinly as you can.
3. Meanwhile, heat the 3 tbsp olive oil over medium-high heat in a deep sauté pan. Once hot, add the fennel with a pinch of salt. Cook for 4 to 5 minutes, stirring occasionally, until some edges start to brown.
4. While the fennel cooks, pour the canned tomatoes with their juices into a bowl. Crush them with your hands. Set aside. Also, crumble the sausage with your hands if you have time (if it doesn't crumble, slice it).
5. Add the garlic to the fennel and cook for 1 minute, stirring frequently. Now add the red pepper flakes and cook, stirring frequently, for 30 seconds.
6. Add the crumbled or sliced sausage into the pan. Break it up with a wooden spatula as it cooks, and cook for 4 to 6 minutes, or until browned and a bit crisp (the exact cook time will depend on the brand of sausage used).
7. Add in the crushed tomatoes with juices, a generous amount of black pepper, and the nutritional yeast. Stir to coat and reduce the heat to maintain a simmer for 15 minutes, stirring occasionally, until the sauce thickens. If desired, smash some of the tomatoes with the back of a wooden spoon from time to time.
8. After you add the tomatoes and the pasta water is boiling, salt the water generously (about 1 ¾ tablespoons kosher salt or 2 ½ to 3 teaspoons sea salt). Add the pasta and cook for the minimum amount of time for al dente pasta according to the package instructions. Before draining, reserve 1 cup (240 mL or 2 soup ladles) of pasta water. Drain the pasta but do not rinse it.
9. Add the baby spinach to the tomato sauce and cook briefly until it just starts to wilt. Add in the hot cooked pasta plus ¼ cup to 1 cup (180 to 240 mL) of the pasta water. Turn the heat to medium-high and toss the pasta vigorously with tongs until it's well-coated in the sauce and is al dente, 1 to 2 minutes. Add more pasta water as needed.
10. Season to taste with salt and pepper. Off the heat, garnish with basil or parsley.