Pat steak dry and sprinkle all over with salt and pepper. Put on rack of a broiler pan and broil 2 to 3 inches from heat. Turning over once, 6 to 8 minutes total for medium-rare. Transfer to a cutting board and let stand 5 minutes.
While steak broils, cut baguette into 4 sections, then halve each horizontally. Spread boursin over bottom halves of bread.
While steak stands, heat 1 tablespoon oil in a 12 inch heavy skillet over moderately high heat until hot but not smoking, then saute romaine in 2 batches, turning over once, until browned in spots and slightly wilted, about 2 minutes per batch. Add remaining tablespoon oil to skillet between batches. Transfer as sauteed to cutting board with meat. Cut off and discard core end of romaine, then lay romaine on top of boursin.
Cut steak across the grain into thin slices and divide among sandwiches.
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