Heat the olive oil in a large deep skillet over medium-high heat. Add the sliced sausage in a single layer and cook for 3 to 4 minutes, turning the pieces halfway through, until they pick up some golden-brown color on both cut sides. That browning is flavor. Transfer the sausage to a plate and leave any rendered fat in the pan.

Reduce the heat to medium. Add the minced garlic to the pan and cook for 30 seconds, stirring constantly so it does not burn. You are just blooming the garlic in the fat from the sausage, which builds the base of the whole dish.

Pour in the diced tomatoes with all their juices and the chicken broth. Stir in the Italian seasoning, black pepper, and red pepper flakes if using. Bring to a gentle simmer.

Add the drained white beans and the browned sausage back into the skillet. Stir everything together and let it simmer uncovered for 8 to 10 minutes, stirring occasionally, until the broth reduces slightly and the sauce thickens enough to coat a spoon. Taste and add salt if needed, keeping in mind the sausage and broth both carry salt.

Remove the skillet from the heat. Add the baby spinach all at once and stir it into the hot mixture. The residual heat will wilt the spinach in about 60 to 90 seconds without turning it dull. Stir just until wilted and bright green.

Spoon into bowls and top each serving with grated Parmesan. Serve with crusty bread on the side to soak up the brothy tomato sauce from the bottom of the bowl.