Heat 2 Tbsp. plus 1½ tsp. peanut or vegetable oil in a large wok or nonstick or carbon-steel skillet over medium. Cook ½ small red or yellow onion, cut into ½”-thick wedges, and 3 Tbsp. finely chopped lemongrass, stirring often, until onion is starting to soften and lemongrass is starting to brown, about 2 minutes.
Add 1 large Fresno chile or jalapeño, quartered, seeds removed, finely chopped, 1 Tbsp. finely chopped garlic, 1½ tsp. Madras-style curry powder (or ¾ tsp. ground coriander, ½ tsp. garam masala, and ¼ tsp. ground turmeric), ¼ tsp. sugar, ⅛ tsp. ground turmeric (if using), and ½ tsp. Diamond Crystal or ¼ tsp. Morton kosher salt; cook, stirring, until fragrant, about 30 seconds. Add 10 oz. oyster mushrooms, torn into bite-size pieces no larger than 2″ (leave smaller pieces intact), and cook, stirring, until coated. Increase heat to high and cook, stirring often, until firm-tender, 2–3 minutes. Add 4 oz. sugar snap or snow peas, ends trimmed, strings removed, and cook, stirring, until warmed through and glistening, about 1 minute. Add 1 Tbsp. vegan or regular fish sauce and cook, stirring, 30 seconds. Taste stir-fry and season with more salt if needed. Immediately transfer to a platter to serve.