Heat olive oil in a 4+ quart lidded pot on medium-high (enamel cast iron is ideal).
While heating, chop onion into ½-inch slices.
Sear sausage links on all sides, until dark brown but not cooked through, then set aside.
Sauté onion in pot until just tender (2-4 minutes), sprinkling with salt.
Press or mince garlic cloves and add to sauté, stirring frequently.
Add fennel, black pepper, anchovy paste, and red pepper flakes shortly after garlic.
As soon as garlic starts to toast, add in the diced tomatoes and crushed tomatoes, along with all liquids from the cans, and tomato paste. Once fully incorporated, add brown sugar and return sausage to sauce, reducing the heat to low and fastening the lid.
Let simmer 1-5 hours, stirring on occasion. You can also transfer the sauce to a crockpot for 4-6 hours. The sauce should start to darken in color after 2 hours.
Salt to taste.
Remove sausage from sauce, and serve over hearty pasta, such as rigatoni or penne.
Top with Parmesan.
