2) Meanwhile, heat the milk in a small saucepan, but don't boil it. In a large pot, melt 90g of butter and add the flour. Cook over low heat for 2 minutes, stirring with a whisk. Still whisking, add the hot milk and cook for a minute or two more, until thickened and smooth.
3) Off the heat, add the Gruyere, Cheddar, 1 tbsp salt, the pepper and nutmeg. Add the cooked macaroni and lobster and stir well. Place the mixture in 6 to 8 individual gratin dishes.
4) Melt the remaining 30g of butter, combine with the fresh breadcrumbs and sprinkle on the top. Bake for 30 to 35 minutes, or until the sauce is bubbly and the macaroni is browned on the top. Serve warm.