Place a large Dutch oven or heavy bottom saucepan over medium-low heat. Render some fat from the ground beef and sausage, allowing a thin layer to form on the bottom of the pan while using a spatula to break the meat into small pieces, 1-2 minutes. Turn the heat up to medium and brown the meat until it’s no longer pink, 3-5 minutes. Drain through a colander and discard the fat. Set the meat aside.
Wipe down the pot briefly to remove any excess fat, but don’t clean it. Heat the olive oil over medium heat, then add the onion along with a pinch of salt. Cook for 3-5 minutes, stirring, until soft. Add the garlic and cook for an additional minute while stirring.
Return the meat to the pan. Add the tomato paste, crushed and diced tomatoes, basil, oregano, and brown sugar, stirring to combine. Bring to a simmer, then reduce the heat to low and cover, leaving the lid slightly ajar to let steam escape. Simmer, stirring occasionally, for 60-90 minutes (longer will give the flavors more time to mingle, but 60 minutes will do the trick).
Stir in the salt, pepper, and crushed red pepper flakes if using. Taste and adjust seasonings if desired.
Serve over freshly cooked pasta (I like topping with some sliced basil and freshly grated parmesan).
Sauce will keep in an airtight container in the refrigerator for up to one week, or in the freezer for up to 3 months.