Sauté the finely diced onion and green pepper strips in olive oil with salt and pepper until peppers are tender-crisp. Remove from skillet and set aside.

In the same skillet, add the ground beef and brown it thoroughly, breaking it apart, until all moisture is cooked out. Drain excess fat if needed.

Add the diced onions back to the beef. Add ketchup, Worcestershire sauce, dark soy sauce, molasses (if using), apple cider vinegar, garlic powder, onion powder, smoked paprika, and cayenne (if using). Simmer for 10-15 minutes.

Make a cornstarch slurry by whisking the cornstarch into the beef stock. Stir the slurry into the beef mixture and cook for 2-3 minutes as the sauce thickens and becomes glossy.

Simultaneously, in a separate heavy pan, melt 2 tablespoons of butter. Add the thinly sliced yellow onions for caramelizing. Cook on low for 20-30 minutes, stirring occasionally, until deeply golden brown and sweet. Set aside.

Split the brioche buns. Butter the cut sides. Place cut-side down in a hot skillet and toast until deeply golden brown, about 2-3 minutes.

To assemble, lay slices of the cheese blend over the hot beef mixture and allow it to melt completely. Broil for 1 minute for a deep melt.

Take a bottom toasted brioche bun. Spoon a massive, messy portion of the saucy beef and cheese mixture onto it. Top with a generous amount of the green peppers and caramelized onions. Place the top toasted bun on, and serve immediately with extra napkins!