In a medium bowl, whisk the olive oil with the lemon juice. Add the watercress, cabbage and onion, season with salt and pepper and toss.
Spread the roast beef slices on a work surface in pairs, overlapping them slightly along the sides, so you have eight 7-by-8-inch rectangles. Season the roast beef with salt and pepper. Mound the watercress salad on the meat and roll into tight cylinders. Trim the ends and cut the rolls into thirds. Sprinkle with salt, dot with pesto and serve.
The rolls can be refrigerated for up to 1 hour.
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