How to Make It
In a large, deep skillet, melt 4 tablespoons of the butter. Add one-third of the minced garlic and all of the scallions and cook over moderately high heat for 1 minute. Add the shiitakes and cook until slightly softened, about 2 minutes. Add the chili powder, thyme and 1 teaspoon of pepper and cook over moderate heat, stirring occasionally, until the shiitakes begin to brown, about 5 minutes. Add the stock, wine and soy sauce and bring to a boil. Simmer gently until reduced to 1 cup, about 40 minutes. Stir the cornstarch slurry and whisk into the liquid. Cook, stirring, until slightly thickened, about 2 minutes. Season with salt; keep warm.
Preheat the broiler. Cut the baguettes into thirds and split the pieces lengthwise, taking care not to cut all the way through. In a small saucepan, melt the remaining 4 tablespoons of butter. Add the remaining minced garlic and cook over low heat until fragrant, about 4 minutes. Brush the garlic butter on the baguettes and sprinkle lightly with salt. Broil the bread, cut side up, for about 1 minute, until golden.
Preheat a grill pan. Brush the tenderloin slices with oil; season with salt and pepper. Grill the meat in batches over high heat, turning once, until charred and medium-rare, about 5 minutes. Dip the meat in the mushroom sauce and arrange on the baguettes. Spoon the mushroom sauce on top, close the sandwiches and serve.