In a toaster or under a grill, toast the bread slices on both sides until dark brown. Leave to cool, then pulse in a food processor to make rough crumbs.
Heat half the butter in a saucepan and cook the onion gently for about 10 mins until just starting to brown and turn sticky. Add the bay leaf and thyme, and pour over the milk. Mix in the spices and season with a sprinkling of salt and pepper. Bring to the boil and simmer gently for 5 mins.
Remove the herbs, then stir in all but a handful of the crumbs and cook over a low heat, simmering gently, until thickened. For a very smooth sauce, blitz with a hand blender in the pan. Can be chilled for up to 2 days or frozen for up to 2 months, then reheated to serve.
While the sauce simmers, melt the remaining butter in a small frying pan and cook until sizzling and nut-brown. Throw in the remaining crumbs and cook in the butter until really crisp, then turn off the heat.
Add the cream to the sauce and heat for a few mins more, then tip into a serving dish. Spoon over the crisp crumbs and any remaining nutty butter to serve.