Melt 1 tablespoon butter in a large saute pan over medium heat, add the onions and salt and cook until brown and caramelized. Add the garlic and cook 1 minute. Deglaze with the wine and reduce by half.
Blanch the beans in salted boiling water, plunge into ice water and drain. Toss them in the pan with the onions, then transfer to a 9 by 13 casserole.
For the sauce, melt the remaining 2 tablespoons butter in a saucepan, whisk in the flour and cook over medium heat until golden. Slowly whisk in the milk and cook until thickened. Add the mustard, cheddar and 2 tablespoons Parmesan and stir until melted. Season with salt, pepper and hot sauce.
Pour the sauce over the beans and press into an even layer. Roll the puff pastry on a floured surface to the size of the dish and lay it over, folding the edges over the sides.
Brush with egg wash and sprinkle with the remaining Parmesan and pepper. Set on a baking sheet and bake at 400 degrees F for 20 to 25 minutes, until golden.