How to Make It
In a medium saucepan, heat 1 tablespoon of the oil. Add the carrot, parsnip and onion and cook over moderate heat, stirring occasionally, until crisp-tender, about 3 minutes. Add the water and butter, season with salt and pepper and cook until the vegetables are tender and the liquid is nearly evaporated, about 5 minutes. Stir in the mirin, soy sauce and Szechuan peppercorns and keep warm.
In a large nonstick skillet, heat the remaining 1 tablespoon of canola oil until nearly smoking. Season the tuna with salt and pepper and cook over high heat, turning once, until browned on the outside but still pink in the center, about 5 minutes. Transfer to a cutting board.
Add the garlic and spinach to the skillet, season with salt and pepper and cook over high heat, stirring, just until wilted. Mound the spinach on plates. Thinly slice the tuna and arrange over the spinach. Spoon the root vegetables and any juices on top, sprinkle with the parsley and serve.