Heat a large straight-sided skillet over medium-high. Swirl in oil. Add onion and 1 teaspoon salt; cook, stirring, 1 minute. Add zucchini; cook until beginning to soften, about 2 minutes. Add salsa, chiles, and ¼ cup water; bring to a boil. Stir in couscous, then top with shrimp in an even layer. Reduce heat to low, cover, and cook 5 minutes.
Sprinkle with peas, cover, and cook until shrimp are opaque and couscous has absorbed all of liquid, 1 to 2 minutes more. Serve with sour cream, sliced radishes, and more salsa.
