At its most basic, all you need is coconut milk, Thai curry paste, and salt. Heat about ¼ cup coconut milk in a sauce pan over medium heat and add in a few tablespoons curry paste (red, green, yellow, masman….they are all good and will keep in your fridge for months). Mix the paste in and let simmer for a few minutes over low heat. Add the rest of your coconut milk in (2 cans total, or 3 to 4 cups) and stir. Add salt to taste and you are ready to go.
From this stage, you can add chopped veggies, meat, tofu, seafood etc. I like eggplant, bamoo shoots, carrots, cauliflower, lotus root, potatoes (sweet or regular), green beans, onions….At the end, before serving, a garnish of torn basil leaves is traditional and delish.
If you want to get fancier, before you add the substance of your curry, you can enhance the sauce itself. Add in lemon grass, kafir lime leaves (I have also used peels of lime zest), galangal, and ginger. You can add fresh chiles if you like it spicier.
You could turn the sauce into a soup, adding stock (veggie, chicken, seafood) to thin it out. You could then add veggies, meat and rice noodles for a traditional tom-kha type of soup, or simmer chunks of butternut squash or pumpkin in the broth and then puree the whole thing, garnishing with some fresh sautéed shitake mushrooms or chopped cilantro.
Thai curry paste, btw, makes a great starting point for lots of yummy sauces. I always have 2 or 3 varieties handy for salad dressings, dipping sauces, soups, marinades, etc. Because there is such a great combination of ingredients already in the paste, it really boosts the flavor of any dish.