Add olive oil to a large deep skillet or braising dish, and set over medium-high heat. Transfer in sausage and poke 3 or 4 times on both sides with the tip of a knife. Cook until sausage is browned and just cooked through, about 5 minutes per side. The heat can be reduced down to medium after the sausage is turned over if the pan is getting too hot. Remove sausage to a plate and let cool. Transfer to refrigerator until needed.
Add diced onion and salt to the skillet, and cook, stirring, until onion turns translucent and starts to brown, about 5 minutes. Add black pepper, chili flakes, oregano, and garlic, and a couple tablespoons of olive oil if there's not already enough fat in the pan. Cook, stirring, for about 30 seconds to 1 minute; pour in broth. Raise heat to high and bring to a boil.
Add beans, reduce heat to medium-high, and simmer, stirring occasionally, until liquid reduces by about half, 10 to 15 minutes.
Meanwhile, cut escarole into 2-inch pieces. Wash in cold water and drain thoroughly.
Slice sausage into 1/4-inch slices, and add to the pan. Reduce heat to medium and cook, stirring, for about 10 minutes. Raise heat to high and bring to a boil, then stir in greens.
Cook, stirring. for about 3 to 10 minutes depending on how tender you want the greens. Reduce heat to medium-low and taste for seasoning. Adjust if needed.
Garnish with freshly chopped Italian parsley, more chili flakes, and another drizzle of olive oil if desired. Use a microplane to zest some of the lemon over the dish, then cut lemon into wedges.
Serve in bowls topped with freshly grated Parmesan and lemon wedges on the side.
