Heat the olive oil in a large skillet over medium-high heat. Add the diced onion and cook for 3 to 4 minutes, stirring occasionally, until it softens and starts to turn translucent around the edges. Add the minced garlic and stir for another 30 seconds until fragrant. Do not let the garlic brown or it will turn bitter.

Add the ground turkey to the skillet. Break it apart with a wooden spoon into small, crumbled pieces as it cooks. You want it cooked through with no pink remaining, which takes about 6 to 7 minutes over medium-high heat. Drain any excess liquid from the skillet so the stew does not end up watery.

Transfer the cooked turkey and onion mixture to your slow cooker. Add the drained cannellini beans, the full can of diced tomatoes with their liquid, and the chicken broth. Stir in the Italian seasoning, smoked paprika, red pepper flakes if using, salt, and black pepper. Give everything a good stir so the seasoning is evenly distributed.

Put the lid on and cook on LOW for 5 to 6 hours or on HIGH for 2 1/2 to 3 hours. About halfway through, use the back of a spoon to gently mash a small portion of the beans against the side of the insert. This releases their starch and thickens the broth without any cornstarch or flour needed.

In the last 20 minutes of cooking, lift the lid and stir in the fresh baby spinach. It will look like a lot but it wilts down quickly. Replace the lid and let it finish. The spinach should be fully wilted but still bright green when you serve it. If it has gone dark and drab, it went in too early or cooked too long.

Taste the stew and adjust salt if needed. Ladle into bowls and top with grated Parmesan if you like. Serve with crusty bread, a side salad, or just on its own. For meal prep, let the stew cool completely before dividing into containers.