Mix mayonnaise and whole grain mustard until well combined. This sauce can be prepared up to 24 hours in advance.
Preheat your griddle on high heat with a drizzle of olive oil.
Roll the ground beef into 1/4 lb balls. Sprinkle them liberally with Blackstone Blackened Steak Seasoning.
Place the red onion wedges on the griddle and cook until each side is caramelized and browned.
Add butter to the griddle. Place the beef balls on the griddle and press down with a burger press or spatula (using parchment paper between the spatula and meat can prevent sticking).
Flip the burgers after 3-4 minutes, ensuring each side is crispy. Add blue cheese crumbles on top of each patty and stack them to create double cheeseburgers.
Toast the hamburger buns in the butter and meat juices on the griddle.
On a plate, place the bottom bun, spread sauce on both sides of the bun, then layer with lettuce, tomato slices, caramelized red onion, burger patties, extra blue cheese crumbles, and crispy onion straws. Top with the other half of the bun.
