Heat olive oil in a large skillet over medium heat. Add bacon; cook, turning often, until crisp and brown, about 10 minutes. Drain on paper towels. Crumble when cool; set aside.

Add onion to same skillet; cook and stir until translucent, about 5 minutes. Stir in chicken, sausage, garlic, thyme, bay leaves, and black pepper; cook until chicken browned, 5 to 8 minutes. Stir in tomato paste.

Transfer chicken mixture to a slow cooker. Stir in great Northern beans, diced tomatoes, and bacon; cover the cooker. Cook on Low until cassoulet thickened and chicken very tender, 4 to 5 hours. Sprinkle with parsley before serving.