In a large nonstick skillet, heat the oil over medium-low heat. Add the onion and sauté until limp but not browned, about 4 minutes. Meanwhile, blanch the lima beans and corn in boiling water to cover for 3 minutes. Drain and set aside.
Add the cucumber and bell pepper to the skillet and sauté, stirring, until both are tender, about 6 minutes. Add the corn and lima beans, and continue to cook, stirring occassionally, for another 2 to 3 minutes.
Taste and add salt (if using) and pepper. Drizzle with lemon juice and stir in the basil. Serve at once.