You can use yellow or brown onions.
You can use vegetable broth or fish or shrimp stock.
If you can't find ground fenugreek, you can replace it with ½ teaspoon of mustard powder or yellow mustard seeds.
The cayenne adds heat; as written, this is medium-mild curry; you can increase the cayenne to make it spicier.
If you want to use fresh spinach, use ½lb, remove any tough stems and finely chop. You will need to give it 3-4 minutes extra cooking time to allow it to wilt down fully.
The shrimp you use in this recipe is totally up to you. You can use large or small, fresh or frozen. The only thing to remember is they must be peeled and raw. If using frozen, ensure they are fully defrosted before using.