In a medium saucepan, heat the oil over medium heat. Add the zucchini, onion and salt to the pan and cook, stirring, until the onion is translucent and the zucchini is tender, about 5 minutes. Lower the heat to medium low, add the peas and carrots, corn and edamame and cook, stirring, for about 1 minute. Transfer the mixture to a large bowl and let cool. Toss with the tomatoes and lemon juice. Season with black pepper.