How to Make It
In a medium saucepan, cover the potatoes with water. Add the halved serrano, the mint stems and a generous pinch of salt and bring to a boil. Cook over moderate heat until the potatoes are tender, about 10 minutes. Drain the potatoes and let cool slightly; discard the chile and mint stems. Cut the potatoes in half.
In a very large skillet, heat 2 tablespoons of the oil. Add the radishes and a generous pinch each of salt and pepper and cook over moderately high heat until just softened, 1 to 2 minutes. Fold in all of the romaine and the sliced serrano. Add the broth and cook over moderate heat, tossing, until the lettuce is just wilted and the radishes are crisp-tender, about 4 minutes. Stir in the potatoes and the chopped mint and season with salt and pepper; transfer to a bowl and keep warm.
Wipe out the skillet and melt the butter in the remaining 2 tablespoons of oil. Season the halibut with salt and pepper and cook over moderate heat until the butter is golden and the fish is browned on the bottom, 3 minutes. Turn the fish. Add the cumin to the butter and cook, spooning the butter over the fish until just cooked through, about 3 minutes. Transfer to plates; spoon the vegetables alongside. Add the lemon zest and juice to the skillet; season with salt and pepper. Spoon the brown butter over the fish and vegetables and serve.