Bring a large pot of heavily salted water to a boil and prepare an ice water bath by filling a bowl halfway with ice and water; set aside. Meanwhile, make the dressing.
Add shallot, lemon zest, olive oil, lemon juice and and measured salt and pepper in a medium, nonreactive bowl and add the lemon juice. While whisking constantly, slowly drizzle in the oil and whisk until evenly combined. Set the dressing aside.
Add the green beans to the boiling water and cook until crisp-tender, about 3 to 4 minutes. Drain and transfer to the prepared ice water bath. When the beans are chilled, drain again and thoroughly pat dry with paper towels.
Place the beans, tomatoes, and parsley in a large bowl, pour in the dressing, and toss to coat thoroughly. Taste and add more salt and pepper as needed.
This salad can be made up to 2 hours ahead, covered, and refrigerated. Toss briefly to recombine the flavours just prior to serving.