One large skillet is all you need to make this easy and cheesy omelet sandwich for four. Cut into quarters, then stuff into a warm baguette with sliced ham for a weeknight bistro dinner in less than 25 minutes.
Preheat the oven to 350˚ F. Whisk the eggs in a large bowl with 1/2 teaspoon salt and a few grinds of pepper; whisk in the scallions. Set aside. Melt the butter in a large ovenproof nonstick skillet over medium heat. Add the spinach, season with salt and pepper and cook, tossing to wilt, about 1 minute. Add the eggs to the skillet and move the spinach evenly around with a wooden spoon. Cook until the edges are slightly set, about 1 minute.
Transfer the skillet to the oven, along with the baguette pieces. Bake until the bread is toasted and the omelet is just cooked through but not browned, 4 to 6 minutes. Arrange the cheese slices over the omelet during the last minute of cooking to melt.
Slide the omelet out of the skillet onto a cutting board. Spread the mayonnaise-mustard blend on one side of each baguette piece. Cut the omelet into 8 wedges. Fill each baguette piece with 2 omelet wedges and 2 slices of ham. Serve with radishes and olives.