This is comfort food with a posh name and a good recipe for using up any weary mushrooms you have in the fridge - or indeed mushroom stalks left over from a recipe that requires only the caps. You'll need a gadget to mince or chop the mushrooms very finely; other than that this is very simple, and the mushroom mixture freezes well. Tastes in garlic and seasoning and fats vary so much that I don't stipulate quantities. If you hate garlic, of course, leave it out; your Duxelles will be just as good.
You could top the toast with a thin slice of cheese and toast again before adding the mushrooms. Or place cheese on top of the mushrooms and then toast. Or add bacon or grilled sliced tomatoes.