How to Make It
Preheat the oven to 425°. Spread the bacon in a single layer on a large rimmed baking sheet. Top with another baking sheet and bake for 10 minutes, or until just barely cooked. Remove the top sheet and bake the bacon until browned, 8 minutes longer. Transfer to paper towels to drain, then cut into 1-inch pieces.
Meanwhile, in a large saucepan of boiling salted water, cook the potatoes until tender, about 12 minutes. Drain and transfer to a bowl; let cool slightly.
In a small bowl, whisk the vinegar with the mustard. Whisk in the olive oil and season with salt and pepper. Add half of the dressing to the potatoes along with the bacon and half of the tomatoes; season with salt and pepper. In another bowl, toss the lettuce, scallions and remaining tomatoes with the remaining dressing and season with salt and pepper. Transfer the greens to 4 plates, top with the potato salad and serve.