Spicy, smoky and tangy, this grilled tomato ketchup may convince you to skip the store-bought variety and opt instead for this fresher, sweeter ketchup recipe forevermore. Grilled onions and bell peppers add even more of that charred flavor to the burger, making this a grilled triple threat.
For the burgers: Sprinkle the onions and peppers with 1/2 teaspoon salt and toss with 1 tablespoon of the oil. Grill the onions and peppers until well-charred and softened, flipping once, about 6 minutes. Remove and set aside. Wipe off the grill to cook the burgers.
Divide the ground beef into four 6-ounce patties, 4 inches wide and 1-inch thick. Brush the burgers with the remaining oil and sprinkle both sides with salt and black pepper. Grill the burgers until visibly cooked more than halfway up the bottom, about 4 minutes, then flip and grill 4 minutes longer for medium rare. Transfer to a cutting board or plate to let rest for 5 minutes.
Place the burgers on the buns and top each with 2 tablespoons of the ketchup, 3 or 4 pepper strips and 4 or 5 onion rings. Serve.