Grill the fish slowly over a distant flame until it's nicely browned.
Literal
Fish [topic-は] distant-fire [with-で] slowly, nicely-browned grill-up-[polite-ます].
Cooking instruction. 遠火 ('distant fire, indirect heat') contrasts with 近火 ('close fire, direct heat') — a traditional grilling technique for even, gentle cooking. こんがり is a mimetic for nicely browned, crispy-golden texture. 焼き上げる ('to grill to completion') uses the ~上げる suffix for completion. Classic Japanese cooking terminology.