Wash and dry all produce. Adjust rack to middle position and preheat oven to 400 degrees. Heat a large pan over medium-high heat (use an ovenproof pan if you have one). Combine beef, panko, Parmesan, meatloaf mix, and ketchup in a medium bowl. Season generously with salt and pepper. Form mixture into two 3/4-inch-thick patties.

Once pan is hot, add a drizzle of oil to it. Add burgers and cook until nicely browned on bottom, 2-3 minutes. Flip burgers, then transfer pan to oven. Bake until burgers reach desired doneness, 6-8 minutes. TIP: If your pan is not ovenproof, transfer burgers to a baking sheet before putting them in the oven.

Trim root and stem ends from onion and peel away skin. Cut off a thin sliver from the middle of the onion at the cheek (fattest part), then place on your cutting board with cut side down (the cut you made will help the onion stay steady). Slice onion crosswise into thin rounds, then separate rounds into rings. Cut lemon into wedges.

Split buns in half crosswise. Place on a baking sheet and toast until golden, 3-5 minutes.

Add lettuce, a squeeze or two of lemon, pepitas, and 1 TBSP olive oil to another medium bowl and toss to combine. Season with salt and pepper.

Fill buns with burgers, then spread 1 TBSP barbecue sauce onto each (you'll have some sauce leftover). Top with a few lettuce leaves from salad and as much onion as you like. Serve with remaining salad on the side.