Cook the pasta in a pot of boiling water.

Meanwhile, in a skillet, sauté the thinly sliced (or finely chopped) onion in olive oil. Add the diced red bell pepper and cook for 2 minutes over low heat, stirring. Add the tomato sauce and a small ladle of pasta cooking water. Cover and cook for about 5 minutes, then add the cannellini beans. Finish cooking and remove from heat.

Drain the pasta and toss it with the sauce in the skillet to combine the flavors.

Season with salt and serve.