House-butchered prime beef seared under a 1,600 degree broiler is the key to Flemings juicy steaks but thats not enough to keep the pricey chain this crowded every night. Maybe its the un- steakhouse-like attention to seasonal fish, with buttery sole or New Zealand king salmon. Maybe its the well- executed sides of colossal onion rings and baked potatoes. Or maybe its a wine list that quenches sophisticated Main Line palates with 100 wines by the glass and more bottles in the cellar. Like the food, the vibe is expected but pleasant: spirited and sleek without being garish.