Add the olive oil to a large, heavy-bottomed stock pot set over medium-low heat, then add the onions and garlic and cook, stirring, for 5 minutes.
Increase the heat to medium. Add the ground beef and Italian sausage and cook, breaking apart the meat with a spatula, until it is cooked through and no longer pink, 5 to 7 minutes.
Add the tomato paste and fennel seeds and cook, stirring occasionally, until the tomato paste begins to caramelize, about 5 minutes.
Stir in the crushed tomatoes, tomato sauce, dried oregano and brown sugar. Reduce the heat to low, cover the pot and simmer the sauce for 10 minutes.
Remove the lid, taste and season the sauce with salt, pepper and additional brown sugar as desired, then serve.
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