Wash the skillet, and return it to the stove.
Add the chopped, reconstituted dried mushrooms to the onions in the bowl along with a hefty pinch of salt, ground black pepper to taste and the ground beef. Mix well, and form into 4 burgers.
In the cleaned skillet, heat the remaining tablespoon of oil over medium. Season the burgers lightly with salt and pepper, then add them to the skillet. Cook the burgers for 3 to 5 minutes per side depending on the desired degree of doneness (3 minutes for rare and 5 for medium-well).
Transfer the burgers to each of 4 plates, and spoon some sauce over each portion.
Nutrition information per serving: 380 calories; 210 calories from fat (55 percent of total calories); 23 g fat (6 g saturated, 1 g trans fats); 80 mg cholesterol; 13 g carbohydrate; 2 g fiber; 2 g sugar; 28 g protein; 300 mg sodium.