Bring a large pot of water to a boil. Add the green beans and boil for 5-7 minutes until bright green and crisp-tender. Drain in a colander and rinse with cold water until the beans are cool. Pat dry with paper towels and transfer to a large mixing bowl.

Add the cherry tomatoes to the bowl with the green beans. In a separate bowl, whisk together the olive oil, lemon juice, vinegar, and mustard. Pour over the vegetables and toss to coat. Season to taste with salt and pepper. Place the salad in a serving dish and top with the feta cheese. Serve immediately.