Slow Cooker Root Vegetable Stew

Name: Slow Cooker Root Vegetable Stew
Ingredients: 1 large white onion OR 2 leeks (white portion only), chopped; 1 lb. butternut squash, peeled, seeded and chopped; 1 lb. carrots, peeled and chopped; 1 lb. parsnips, peeled and chopped; 1 lb. sweet potatoes, peeled and chopped; 1 lb. Yukon Gold potatoes, peeled and chopped; 2 celery ribs, stems removed and chopped; 6 cloves garlic, peeled and thinly sliced; 3 cups chicken or vegetable broth; 1 bay leaf; 1 Tbsp. fresh sage leaves, finely chopped; 1 tsp. freshly-cracked black pepper; 1/2 tsp. sea salt; 2 cups chopped fresh kale