How to Make It
In a large skillet, melt 2 tablespoons of the butter. Add the mushrooms and season lightly with salt and pepper. Cover and cook over moderate heat, stirring a few times, until tender and browned, 8 minutes. Add the sherry and boil over high heat until reduced by half, 1 minute. Add the demiglace and simmer over moderate heat until slightly reduced, 3 minutes. Stir in the cream and simmer until thickened, 5 minutes. Add the shallot, vinegar and chives and season with salt and pepper.
Heat a nonstick skillet over moderate heat and melt the remaining 1 tablespoon of butter in it. Add the pastrami and cook until warmed through.
Fill the rolls with the pastrami. Top with the mushrooms and cheese and serve.
Demiglace, the rich concentrated sauce, is available at specialty markets.