Juicy Grilled American Lamb Burger with Caramelized Onion and Lemon Relish

Name: Juicy Grilled American Lamb Burger with Caramelized Onion and Lemon Relish
Ingredients: 3 tablespoon extra-virgin olive oil; 1 red onion, cut into thick slices; 1 fennel bulb, cut into 1/4-inch slices; dash Salt and pepper, to taste; 1 teaspoon minced fresh rosemary; 2 Zest of 2 lemons (pith removed, zest finely chopped); 1 teaspoon honey; 1 pinch red chili flakes; 2 pound ground American Lamb; dash Salt and freshly ground black pepper; 8 dinner rolls; Mayonnaise, to taste; Arugula leaves, to taste