Halve the leek lengthwise and cut crosswise into 1/2-inch pieces.

Wash the leek in a bowl of cold water and transfer to a sieve to drain.

Chop the onion and cut the red bell pepper into 1-inch pieces.

Mince the garlic.

In a 12-inch non-stick skillet, cook the leek, onion, bell pepper, and garlic in 1 tablespoon of olive oil over moderate heat, stirring occasionally, until the onion is softened and the pepper is tender, about 15 minutes.

Transfer the vegetables to a bowl.

Pat the chicken dry and cut crosswise into 1/2-inch-thick strips.

Season the chicken with salt and pepper and cut the polenta into 3/4-inch cubes.

In the skillet, heat the remaining tablespoon of olive oil over moderately high heat until hot but not smoking and saute the chicken, stirring, until golden, about 3 minutes.

Add the polenta and cook, stirring occasionally, for 1 minute.

Stir in the cooked vegetables, tomatoes, thyme, and parsley and simmer, stirring, until heated through, about 2 minutes.

Season the ragout with salt and pepper.