To Make the Burger Sauce: In a small bowl, mix ketchup, mayo, sriracha, mustard, garlic powder and a dash of salt and pepper. Cover and place in the fridge until your burgers are ready.
For the Grilled Onions: In a small skillet over low heat add sliced red onions and butter. Stir to coat onions and let cook on low for about 15 minutes until onions are cooked through and begin to lightly caramelize.
For the Burgers: Heat a grill pan or skillet to medium-high heat. Form four patties with your hands. Salt and pepper each side of your burgers and drizzle with about a tsp of worcestershire sauce each. Transfer burgers to the hot pan. Cook for about 4-5 mins until juices start to show on top of your patties. Flip the burgers over and cook for another couple minutes. Place cheese on top of each patty and let melt. When done, remove from heat.
For the Bread: Lightly butter your bread on each side and place on hot grill pan for about 1 minute on each side until nice and and grilled.
Assemble your Frisco Burger: Slather each slice of sourdough bread with the burger sauce. Top with a cheeseburger patty, grilled onions, tomato and romaine lettuce. Serve with your favorite side dishes. Enjoy!