Sauté the onions in a pot with the butter over medium-low heat until the onions begin to brown. As the onions begin to caramelize they will turn a dark amber color.
Add the garlic and cook for 1 to 2 minutes.
Add the ketchup, lemon juice, Worcestershire sauce, balsamic vinegar, water, tamari, dark brown sugar, Dijon mustard, cayenne, salt, and pepper, stirring to combine.
Raise the heat to medium and simmer until slightly thickened about 30 minutes, stirring occasionally.
Optional: Pour the sauce through a mesh strainer placed over a large bowl using a spatula to press the sauce through. Wipe any excess off the bottom of the strainer and into the bowl.
Peppered Steak
Prepare a grill to cook at medium-high heat or 400 degrees F (204 degrees C), indirect cooking method.
Brush the steak with a light coating of olive oil then season with pepper and salt. Let the steak rest for 10 to 15 minutes.
Sear the steak over direct heat for 2 to 3 minutes, then rotate 45 degrees and sear for another 2 to 3 minutes. Flip over and repeat on the other side.
Move the steak to the indirect side of the grill and close the lid. Continue to cook until the desired doneness. Remove the steak from the grill and allow to rest for 5 minutes.
Slice and arrange on a platter to serve with the onion sauce.