In a sauté pan on med-high heat, coated lightly with oil, add onions, season with salt and pepper. Sautee until caramelized, about 8 minutes and set aside. In same pan on med-high heat and coated with oil, add maitakes, garlic and sauté till crisp, let maitakes sit in pan to brown on one side before flipping. Add 1 tablespoon of naturally brewed soy sauce and cook for an additional minute. Set aside. Again in same pan, coat with oil and sear the beef quickly, season. Add back the onions to heat through then add the maitakes. Meanwhile, mix the Dijon mustard, mayo and peppercorn together. Make sandwiches layering the basil, tomatoes and lettuce. Spread Dijon mustard-mayo on both sides of bread. Top with hot meat mixture.