If making the harissa paste from scratch, do this first or make ahead.
Make the Harissa Aioli- whisking mayo with harissa paste, or spices in a small bowl, set aside.
Make quick pickled red onions and refrigerate them.
Preheat grill to medium-high.
Make the burgers: In a medium bowl, using your hands, combine ground lamb with onion, garlic, spices, salt and pepper, just to combine, being careful to not over mix. Form patties and set aside.
Crisp bacon slices (if using) and set on paper towel.
Slice the cheese.
When the grill is hot, grease the grates and grill lamb patties 4-5 minutes on each side, closing the lid. Cook to the desired doneness. During the last minute of grilling, top with the cheese to melt it and toast buns at the same time.
Assemble. Once cheese has melted, assemble. Spread harissa aioli on both sides of the buns, layer the lamb burgers, optional bacon, arugula, tomatoes and pickled onions and then, the bun top.
Serve immediately!
