Rinse the beans (unless you are using canned), and put them in a bowl with cold water to cover by at least 4 inches. Let soak in a cool place for 8 hours or overnight. Drain the beans, and transfer them to a large saucepan with fresh cold water to cover by two fingers. Bring to a boil, partially covered, and cook the beans about 40 minutes, until tender but not mushy. Turn off the heat and stir in ½ teaspoon salt, then let the beans cool to absorb the cooking liquid. Taste, and adjust the salt if needed.

Bring 6 quarts of water to the boil in the stockpot. Meanwhile, rinse and drain the Swiss-chard leaves. Cut off the stems at the base of the leafy part. Slice the leaves crosswise every 2 inches or so, into long strips.

When the water is boiling, drop in all the cut chard at once, stir, and cover the pot. Bring the water back to the boil, and cook the chard for about 15 minutes, until thoroughly tender—check a piece with a thick middle vein to be sure. Drain the cooked chard well in a colander. Drain the cannellini (and rinse them too, if using canned beans).

Heat ¼ cup of the olive oil and the sliced garlic in the skillet over medium-high heat, stirring frequently, until the garlic is sizzling, about 2 minutes. Drop the spoonful of tomato paste into a clear spot in the pan, stir, and toast it for a minute. Toast the peperoncino in a hot spot too, then pour in the crushed tomatoes and stir everything together.

Bring the tomatoes to a boil, and spill in all the beans. Stir, season well with salt, and heat the beans rapidly, stirring constantly. When they’re simmering, stir in the chard and bring to a boil over high heat. Cook rapidly for a couple of minutes to reduce the liquid, tossing the beans and greens over and over. As the juices thicken, drizzle the remaining 2 tablespoons olive oil all over, toss it in with the vegetables, and simmer for another 2 or 3 minutes, until most of the liquid has evaporated.

Serve right away, or set aside the skillet, covered, and reheat later.