Preheat oven to 400 degrees. Prepare 4 pieces of parchment paper or aluminum foil for papillotes (as described in previous "All Wrapped Up" recipes). Using a sharp knife, remove skin and white pith from the oranges. Cut the segments away from their surrounding membranes. Squeeze any remaining juice from the membranes into a small bowl. Stir in orange-juice concentrate, thyme, soy sauce, garlic and pepper.
Place a piece of tuna on one half of each opened paper heart. Distribute mushrooms and orange sections over the tuna. Spoon orange juice mixture over top. Seal the packages and place them on a baking sheet. Bake for 10 to 12 minutes, or until the packages have puffed. (You may want to open one package to check that the tuna flesh is opaque.) Transfer the packages to individual plates; let each diner open his or her own package. Serves 4. 212 calories per serving: 6 grams fat, 92 mg sodium and 43 mg cholesterol.
