Heat oil in a skillet over medium-high heat. Add turkey, onion, and red bell pepper. Saute until pepper is soft and turkey is no long pink, 6 to 8 minutes.
Combine water, diced tomatoes, tomato sauce, lentils, carrots, barley, bouillon, salt, Italian seasoning, black pepper, and cayenne pepper in a large pot over medium-high heat. Add the sauteed turkey mixture. Bring to a boil. Reduce heat and simmer, covered, until lentils are tender and flavors are blended, about 1 hour.