Stuffed Beef Tenderloin with Blue Cheese Sauce

A favorite market, Russo's, often has nice vacuumed-packed cuts of beef. This time I bought a pack of beef tenderloin ends for around $5.50. It contained two pieces, which I was able to butterfly with a sharp knife.
In a pat of butter and splash of olive oil set over medium high heat, sautee 1/2 a chopped large onion, for 5 minutes.

Saute 8 shiitake mushrooms, chopped up, for an additional 5 minutes.
Reduce heat to medium and continue to cook, stirring often, until veggies are deeply browned, about 10 minutes longer.
Stir in 1 clove of minced garlic and cook for 30 seconds, until fragrant.

Pour in 1/4 cup Madeira wine and scrape up any browned bits. Cook 2-3 minutes until it evaporates. Put mixture in a separate container and set aside to cool.
Pat mixture onto tenderloins. Top with baby spinach leaves. Roll up and tie with butcher's twine.

Drizzle with olive oil to coat, both sides. Salt & pepper.
Preheat oven to 450º.
Brown in a skillet preheated over medium high heat.
Cast iron works well.
Turn to brown all sides. Transfer to an oven dish and roast for 10 minutes.
Allow to cool a couple minutes then snip ties away.
Make a sauce with melted (microwaved) blue cheese (3/4) and butter (1/4). Pour any pan juices into it and mix up. We had it easy in that we just did that to a gifted cheese ball (also with chopped walnuts, which is a nice texture). Top with some sliced scallions (green onions). They make it sooo good!
Though not done here, Alex suggests cooking 3 strips of bacon until crispy, then crumble and also use as topping.
This meal took so much effort, that the taster was already asleep and we didn't bother with sides!