Heat the skillet: Place your skillet over medium-high heat and add the olive oil.
Brown the sausage: Add the sliced sausage to the hot skillet and cook, stirring occasionally, until browned on both sides, about 5-7 minutes. Transfer to a plate, leaving the flavorful drippings in the pan.
Sauté the aromatics: Reduce heat to medium and add the diced onion to the skillet. Cook until softened, about 3-4 minutes. Add the garlic and bell pepper, cooking for another 2 minutes until fragrant.
Build the flavor base: Stir in the tomato paste, dried oregano, dried thyme, and red pepper flakes (if using). Cook for 1 minute, stirring constantly to prevent burning and to toast the spices slightly.
Add the beans and liquid: Return the browned sausage to the skillet. Add the drained white beans, diced tomatoes with their juice, and chicken broth. Stir to combine, gently scraping up any browned bits from the bottom of the pan (that's flavor gold!).
Simmer: Bring the mixture to a simmer, then reduce heat to medium-low. Let it cook uncovered for about 10 minutes, allowing the flavors to meld and the liquid to reduce slightly.
Add the greens: Stir in the chopped spinach or kale and cook until just wilted, about 2 minutes.
Finish with brightness: Remove from heat and stir in the fresh parsley and lemon juice. Season with salt and pepper to taste.
Serve: Ladle into bowls and top with grated Parmesan cheese if desired. Serve with crusty bread for sopping up the delicious broth.