healthy slow cooker recipe

1 tablespoon olive oil - 1 large onion, chopped - 2 cloves garlic, minced - 3 carrots, peeled and chopped - 3 celery stalks, chopped - 2 large potatoes, peeled and diced - 1 red bell pepper, chopped - 1 zucchini, chopped - 1 can (15 ounces) diced tomatoes, undrained - 1 can (15 ounces) black beans, rinsed and drained - 1 can (15 ounces) kidney beans, rinsed and drained - 4 cups vegetable broth - 1 teaspoon dried thyme - 1 teaspoon dried oregano - 1 bay leaf - Salt and pepper to taste - 2 cups fresh spinach, chopped - Fresh parsley, chopped (for garnish)