Zuppa dell'Ortolano (Greengrocer's Soup with Onion, Peppers, Rapini, and Potatoes)

Ingredients

• 1½-2 lb. potatoes
• 1 bunch of rapini, about 1 lb.
• Fine sea salt
• 1 large meaty red bell pepper or 2 smaller ones
• 1 tbsp. vegetable oil
• 3 tbsp. butter
• ⅔ cups chopped onion
• Black pepper ground fresh from the mill
• 2-4 cups meat broth, or a beef bouillon cube dissolved in water, or canned beef broth diluted with 3 parts water