Drizzle a tablespoon or two of oil into a large Dutch oven and place over medium-high heat until shimmery and sizzling hot. Add the Italian sausage (do not break it up yet) and sear until golden-brown on one side, about 2 to 3 minutes. Flip and sear the other side, another 2 to 3 minutes. Once the sausage is lightly browned on both sides, start aggressively breaking it up with wooden spoon. (The goal is to get some delicious caramelization in the bottom of the pan as opposed to just steaming the ground meat in its own liquids.) Using a slotted spoon, transfer the sausage to another bowl, leaving the fat in the pot.
Reduce the heat to medium and add the onions and a big pinch of salt. Cook the onions, stirring occasionally, until soft and translucent, about 5 to 8 minutes. Add the garlic and cook for another 15 to 20 seconds, just until you start to smell its aroma.
Add the red wine and increase the temperature to high. Allow the liquid to cook out at a raucous boil, making sure to scrape the bottom of the pot to release any delicious brown bits as it bubbles.
Add the canned tomatoes and juices to the pot. Add the chicken stock, balsamic vinegar, brown sugar, Parmesan rind, and cooked sausage.
Season with kosher salt — start with about a 1/4 teaspoon — and freshly ground black pepper. Bring the liquid to a boil and add the tortellini. Reduce heat to medium-low and simmer until tortellini is warmed through, about 10 to 15 minutes.
Add the spinach to the pot and stir until wilted. Taste and adjust seasoning. (Remove Parmesan rind if used.) Serve with a drizzle of good olive oil and a dusting of shaved Parmesan