How to Make It
Season the tuna steaks with salt. In a medium saucepan, melt the butter in the olive oil and lime juice. Stir in the minced jalapeño and sliced kumquats. Add the tuna steaks and cook them over moderate heat, turning once, until they're rare, about 4 minutes.
Transfer the tuna steaks to a carving board and slice them 1/3 inch thick, then arrange them on plates. Stir the torn mint into the sauce, spoon over the tuna and serve.