Place the green beans in a large stock pot. Cover the green beans with water 2 inches above the beans. Place on the stove over high heat, and bring to a boil. Once at a boil, lower the heat to medium and simmer for 5 minutes. Strain and set aside.
In a large bowl whisk together the olive oil, vinegar, garlic, dijon, salt, and pepper. Add the warm green beans and onion, toss to coat. Cover with plastic wrap and place in the fridge to chill for 4 hours or longer if you can.
Before serving add the tomatoes and cheese, toss to combine, and serve immediately.