Slow Cooker Jambalaya (Vegan)

1 tablespoon olive oil, or to taste

1 (28 ounce) can diced tomatoes with juice

8 ounces seitan, cut into cubes

8 ounces smoked vegan sausage, cut into 2-inch slices

½ large onion, chopped

½ large green bell pepper, seeded and chopped

3 stalks celery, chopped

1 cup vegetable broth

2 cloves garlic, minced

1 tablespoon miso paste

1 ½ teaspoons Cajun seasoning

½ teaspoon dried thyme

½ teaspoon dried oregano

1 cup rice

1 tablespoon chopped fresh parsley, or to taste (Optional)