This is the sort of supper that will make you look like a real pro in the kitchen, yet nobody needs to know how easy it is to prepare. The combination of red curry paste and coconut milk is a magical one. The salmon gently cooks in the sauce, resulting in supremely tender fish that is as pretty to look at as it is tasty to eat. Serve with steamed brown rice or noodles.
Chop the onion. Quarter the sweet potato lengthwise then cut crosswise into 3/4-inch pieces. Halve the zucchini lengthwise then cut crosswise into 3/4-inch pieces.
In a Dutch oven or large, heavy pot with a lid, heat the olive oil over medium. Add the onion and saute until tender and translucent, about 5 minutes.
While the onion cooks, pull any tiny bones from the salmon (tweezers can be useful for this). Cut into four equal pieces. Set aside.
When the onions are tender, add the coconut milk, water, curry paste, brown sugar, and fish sauce, and stir until the ingredients are blended and the sauce is smooth. Add the sweet potato, stir, and turn the heat to high. When the liquid boils, drop the heat until it simmers, put on the lid, and cook the sweet potato until just tender, about 8 minutes.
Gently turn the salmon over. Add the zucchini to the pot, doing your best to nestle it into the sauce. Return the lid and simmer until the salmon is done (it will be opaque in color and easily pull apart, about 3 minutes more (the time will vary depending on the thickness of the fish and it will continue to cook a bit even once it’s off the heat). Scatter the spinach over the curry and cover again with the lid. The spinach will wilt quickly.
Serve in shallow bowls as is or spooned over steamed rice.
Red Thai curry paste is sold in the small jars in most supermarkets where you'll find other Asian ingredients. Thai Kitchen is a common brand. The level of spiciness varies by brand, so if you are sensitive to heat, start with 1 to 1 1/2 tablespoons and taste before you add the full 2 tablespoons. If you can't find red curry paste, you can substitute green (it's still delicious, but not as pretty).