(*NOTE: If wild porcini sea salt is not available, increase plain salt by equal amount.)
In large bowl, combine Montreal steak seasoning, seasoned salt, wild porcini salt, plain salt, garlic powder and ground black pepper; add 2 tablespoons mayonnaise; mix well; add chopped onions; mix.
(Food service plastic gloves are advised.) Add ground beef; mix thoroughly by hand; form mixture into 8 fairly equal-sized hamburger patties and place on a large plate or platter; set aside, or cover and refrigerate, until ready to dredge in flour.
To prepare dredge mixture, in a shallow dish, mix together 1 cup flour, 1 teaspoon salt and 1/4 teaspoon pepper.
Working with one patty at a time, place patty in flour mixture; turn to coat all sides; place patty back on plate; repeat until all patties are coated. Reserve remaining flour for gravy.
Preheat oven to 350ºF.
To a large skillet, add 2 tablespoons vegetable oil; place over medium-high heat.
When oil is hot, add patties to skillet.
Pan-fry patties until browned on both sides (patties do not need to be fully cooked as they will continue cooking in oven,) about 3 to 4 minutes per side.
Remove browned patties and place in single layer in a 9"x13" or smaller baking dish.
Remove skillet from heat; pour grease out of skillet, retaining about 1 1/2 tablespoons of the grease with the brown bits.
Add 2 1/2 tablespoons of the reserved flour to the grease; using a large, heat-proof spoon or spatula, blend together as much as possible (some of the flour will not be blended and that's okay).
Return to heat; slowly stir in 1 can of beef broth, smoothing any clumps of flour; bring to simmer, stirring constantly to avoid lumps as gravy thickens.
Remove from heat; add 2 teaspoons soy sauce and pinch of freshly ground black pepper; stir well; pour gravy over patties in dish.
Cover dish with aluminum foil.
Bake at 350ºF for 30 minutes. Serve hot. (Serve with mashed potatoes, noodles or rice or over toasted bread.)