Liz's Slow Cooker Chicken Curry

1 large onion, cut into wedges

3 large skinless, boneless chicken breasts, chopped

¼ cup butter, cut into chunks

5 cloves garlic, chopped

4 carrots, peeled and sliced

5 potatoes, peeled and cubed

1 head cauliflower, separated into florets

4 ½ tablespoons curry powder

2 tablespoons ground turmeric

1 tablespoon red chile flakes, or to taste

½ tablespoon ground ginger

salt and ground black pepper to taste

4 cups chicken stock, or as needed

4 tablespoons all-purpose flour, or as needed