Fill a large pot with water, add 1/2 cup salt and heat to a boil.
in the boiling salted water until tender, about 5 minutes, and then transfer to a towel to drain
. Set aside until beans are needed.
In a medium saucepan melt
2 1/2 tablespoons butter
and add the 4 tablespoons flour. Cook, stirring often, at least 5 minutes on low heat. Add the milk
slowly, whisking constantly, until the flour is dissolved.
In a small pan, toast the fenugreek
leaves over medium heat for about 5 minutes, just to bring out their smell and taste. Turn off heat and grind the leaves in a spice grinder
or food processor
. Add the ground leaves to the sauce.
In another pan, over medium heat, add the remaining 2 1/2 tablespoons butter and heat until it begins foaming. Add the mushrooms to the pan and cook until well caramelized, about 10 minutes. Add the shallot to the mushrooms, cook 2 more minutes. Add the vermouth
to the pan and increase the heat to high to quickly cook off the alcohol
. When the pan looks dry, transfer the mushrooms to a paper towel-lined tray and leave to cool.
Once the mushrooms are cool enough to handle, roughly chop
them and add to the sauce along with the thyme sprigs. Let the sauce cook over low heat for about 1 hour, stirring often.
Preheat the oven to 325 degrees F.
Add the beans to a casserole dish
and arrange nicely. Pour over the mushroom sauce and bake in an oven for 15 to 20 minutes until lightly browned.
Remove the casserole from the oven and garnish
with the onion rings.