Wash the clams well under cold running water. If there are any clams which will not shut when tapped on the shell, then discard those as it might not be fit to eat.
To prepare the roux, heat butter in a pan and add the flour when the butter has melted. Keep stirring for 1-2 minutes taking care that the flour and butter does not burn. Remove from heat and keep aside.
In a large pot, place the clams, shallots, parsley, wine and water. Cook on high heat allowing the liquid to boil for a couple of minutes and all the clams have opened.
Remove the clams one by one from the pot and place in another bowl. If there are any unopened ones, allow more time to cook which will open up the shells. But if any remain closed even after cooking a few minutes, then discard those. Sieve the remaining liquid through a muslin cloth to remove any remaining grit.
Return the strained liquid back to heat, bring to boil. Then add the cream slowly and stir well; allow to boil and then add the roux to thicken the sauce. Season with salt and pepper as necessary.
In a separate bowl, mix the remaining butter, cheese and breadcrumbs.
To assemble, place the clams in an oven proof dish, pour the sauce all over the clams and then add the crumb mixture evenly on top. Place in a hot oven or under a hot grill for 1- 2 minutes or till the crust is golden.