Preheat the oven to 250 degrees F (120 degrees C).
Heat 1 tablespoon olive oil in a skillet over medium-high heat. Lightly coat the veal medallions in flour, and season with salt and pepper. Cook in the heated skillet about 5 minutes, to an internal temperature of 160 degrees F (70 degrees C). Remove from the skillet, place in a baking dish covered with foil, and keep warm in the preheated oven until ready to serve.
Heat the remaining olive oil in the skillet over medium low heat, and saute the shallot and mushrooms, scraping up any browned bits, until shallots are tender. Increase heat to medium high, and stir in the Marsala and garlic. Cook and stir until thickened. Mix the chicken broth and demi glace into the skillet, and continue to cook and stir until reduced to about 1/4 cup. Remove from heat, and whisk in the butter until melted. Serve over the veal.