vegan croque-monsieur aka savory french toast

I've had a thing for open-faced croque-monsieurs after I had an amazing one at Tartine in SF. I recently learned that croque-monsieurs include Béchamel sauce along with gruyère. I had this idea to make a vegan croque-monsieur with lots of Béchamel (just margarine, flour, and soymilk) and no cheese. The sauce browns nicely, but stays nice and gooey in all of the holes in the bread. Essentially, this is like a savory French toast, and it's great as a quick snack.

Vegan Béchamel Sauce Ingredients:
2 T. margarine
2 T. all-purpose flour
1/2 to 1 c. soymilk, depending on desired thickness
salt to taste
optional: nutmeg or cloves (ground)
optional: a splash of vinegar (cider or wine or hopped-up honey vinegar) adds some nice tanginess

Melt the margarine, mix in the flour (you know, like making a roux), add some soymilk and seasoning, and stir and cook until it's at a desired consistency. It should be pretty thick and come away from the pan. Spread it on a slice of nice chewy and holey bread and toss it on the frying pan like you're making Texas or French toast.

Then make a sandwich or top it with large chunks of caramelized onions and mushrooms soaked in wine. I bet sun-dried tomatoes would be great too.