Heat remaining 2 Tbsp. oil in a large pot or high-sided skillet over medium-high. Add onions and a generous pinch of salt and stir to coat. Cook, stirring often, until softened, about 4 minutes. Stir in curry paste and cook, stirring often, until darkened in color, about 2 minutes. Add zucchini, eggplant, and green beans and cook, tossing to coat, until vegetables are softened and starting to brown in spots, 5–7 minutes. Pour in coconut milk and ½ cup water and bring to a simmer.