Set a large skillet on the stove over medium heat. Set a medium saucepan on another burner so it is ready for the polenta later.

Add the olive oil to the skillet. When the oil looks loose and shiny, add the chopped onion and 1/2 teaspoon of the salt.

Cook the onion for 4 to 5 minutes, stirring a few times, until it is soft and no longer looks raw.

Add the sliced garlic and the crushed red pepper flakes, if using, to the skillet. Cook for 30 seconds, stirring, until the garlic smells fragrant.

Pour the crushed tomatoes into the skillet. Stir well and cook for 2 minutes so the tomatoes lose their raw canned taste.

Add the drained cannellini beans and 1/2 cup of the water to the skillet. Stir and bring the mixture to a gentle simmer.

Lower the heat under the skillet to medium-low. Let the beans simmer for 8 minutes, stirring once or twice, until the tomato mixture thickens slightly.

Add the chopped kale to the skillet a handful at a time. Stir each handful in before adding the next so the leaves wilt down instead of spilling out.

Cook the kale in the bean mixture for 2 to 3 minutes, until the leaves are tender but still green.

Stir the lemon zest and 1 tablespoon of the lemon juice into the skillet. Taste the bean mixture and add more salt or black pepper if needed, then keep the skillet on the lowest heat while you make the polenta.

Pour the vegetable broth and the remaining 2 cups water into the medium saucepan. Bring the liquid to a boil over high heat.

Slowly whisk the quick-cooking polenta into the boiling liquid. Keep whisking as you add it so it does not form lumps.