When you’re craving a burger but don’t want a gut bomb, swap out the fatty beef for a thick fillet of halibut. It’s just as satisfying, and the mild fish is amenable to whatever flavors you want to throw at it. For this take, I slather the fillets in citrusy mayo and a sweet-savory blueberry relish that puts boring old ketchup to shame. I pile on a shaved fennel slaw for crunch.