In a medium saucepan of boiling water, cook the scallions for 30 seconds; drain. In a bowl, whisk together the beer, soy sauce and sugar; set aside.
Trim each roast beef slice to about 4 by 5 inches. Season with salt. Lay a small cluster of scallion and cucumber slivers on the bottom edge of each slice and roll tightly to form a log; secure with a toothpick along the length of the roll. Repeat with the remaining beef, scallions and cucumber. Using a basting brush, coat the logs with the reserved beer sauce; let stand for 10 minutes.
Meanwhile, in a large skillet, heat the oil over medium-high heat. Add the beef rolls (reserve the sauce) and cook, turning, until the sides are brown, about 5 minutes. Transfer to a plate. Add the reserved beer sauce to the skillet and simmer until syrupy, about 2 minutes.
Remove the toothpicks from the beef rolls; discard. Cut the rolls into quarters. Top with the sauce.
Top the cooked rice with the cilantro and serve with the roll-ups.