Prepare the ingredients:

If you’re grilling, preheat your grill to maintain a temperature of 450-500°F. Oil the grill grates. Wash and dry the fresh produce. Peel and thinly slice the onion. Halve the buns. Cut off and discard the root end of the lettuce; separate the leaves. Quarter and core the apple; thinly slice lengthwise. Combine in a large bowl.

Caramelize the onion:

In a medium pan (nonstick, if you have one), heat a drizzle of olive oil on medium-high until hot. Add the sliced onion; season with salt and pepper. Cook, stirring occasionally, 7 to 9 minutes, or until lightly browned and slightly softened. Add the Worcestershire sauce (carefully, as the liquid may splatter) and crème fraîche. Cook, stirring frequently, 30 seconds to 1 minute, or until combined. Transfer to a bowl. Season with salt and pepper to taste. Cover with aluminum foil to keep warm.

Form & cook the patties:

Place the beef and breadcrumbs in a bowl; season with salt and pepper. Gently mix to combine. Using your hands, form the mixture into two 1/2-inch-thick patties; transfer to a plate. 

STOVE: Rinse and wipe out the pan used to cook the onion. In the same pan, heat a drizzle of olive oil on medium-high until hot. Add the patties; cook 4 to 5 minutes per side (flipping carefully, as the oil may splatter), or until browned and cooked through.
GRILL: Lightly oil the patties. Grill 6 to 7 minutes per side, or until browned and cooked through. 

Transfer to a plate and cover with aluminum foil to keep warm.

Make the salad:

To make the dressing, in a bowl, combine the creamy mustard sauce and half the vinegar (you will have extra vinegar). Whisk in a drizzle of olive oil until thoroughly combined; season with salt and pepper. Just before serving, to the bowl of prepared lettuce and apple, add the peppers and enough of the dressing to coat (you may have extra dressing); season with salt and pepper. Toss to combine; season with salt and pepper to taste.

Toast the buns & serve your dish:

STOVE: Wipe out the pan used to cook the patties. Add the buns, cut side down, and toast on medium-high, 30 seconds to 1 minute, or until lightly browned. Transfer to a work surface.
GRILL: Grill the buns, cut side down, 30 seconds to 1 minute, or until lightly browned. Transfer to a work surface.

Assemble the burgers using the toasted buns, cooked patties, and caramelized onion (you may have extra onion). Serve with the salad on the side. Enjoy!