The size of the red bell pepper doesn’t matter as much as the overall volume; 1 medium bell pepper is about 8 ounces. Jarred crushed Calabrian chiles are available in the condiment section of some specialty markets or online. You can use other canned beans, such as Great Northern or chickpeas, in place of the cannellini beans. You can use chicken broth for the vegetable broth if you prefer; note that vegetable broth salt levels vary dramatically from brand to brand, so be sure to season with salt and pepper to taste before serving.

Adjust oven rack 6 inches from broiler element and heat broiler. Toss bell pepper and cherry tomatoes with 2 tablespoons oil in bowl, then spread in even layer, skin side up, on rimmed baking sheet. Broil until well charred, 8 to 12 minutes. Transfer vegetables and any juices to food processor.

Add whole basil leaves, Parmesan, almonds, Calabrian chiles, tomato paste, vinegar, garlic, and ½ teaspoon salt and process to coarse paste, 15 to 20 seconds, scraping down sides of processor bowl once halfway through processing. Season with salt to taste. (Pesto can be refrigerated for up to 24 hours.)

Heat remaining 2 tablespoons oil in Dutch oven over medium heat until shimmering. Add onion and remaining ¼ teaspoon salt and cook until softened, about 5 minutes. Stir in beans, pesto, and broth, increase heat to medium-high, and bring to boil. Reduce heat to medium-low and simmer until thickened slightly, about 10 minutes. Season with salt to taste, then sprinkle with chopped basil. Serve with extra Parmesan and extra Calabrian chiles.