1 medium cucumber—peeled, halved lengthwise, seeded and sliced crosswise
4 large scallions, minced
1/2 green bell pepper, cut into strips
In a small bowl, combine the lemon juice with the mustard and garlic. Whisk in the oil in a thin stream. Season with salt and pepper, then add the parsley.
In a large saucepan of boiling salted water, cook the beans until just tender, about 4 minutes. Using a slotted spoon, transfer the beans to a colander and refresh under cold running water. Drain and pat dry. Cut the beans into 2-inch lengths. Add the beets to the boiling water and simmer over moderate heat until tender, about 40 minutes; drain. Peel and cut them into 2-inch chunks.
In a large bowl, toss the beans with the beets, lettuces, cucumber, scallions and bell pepper. Add a few tablespoons of the vinaigrette and toss to coat. Serve with the remaining vinaigrette on the side.