How to Make It
In a baking dish, whisk 1/4 cup of the mustard with 2 tablespoons of the oil and a pinch each of salt and pepper. Stir in 1/4 cup of the cilantro. Add the tuna and turn to coat. Cover and refrigerate for 30 minutes.
In a large saucepan, cover the taro with water and bring to a boil. Add a large pinch of salt and simmer over moderate heat until very tender, about 25 minutes. Drain well and return to the saucepan. Add the cream, milk and butter and mash. Season generously with salt and pepper; keep warm.
In a small bowl, whisk the remaining 1/4 cup of oil and 2 tablespoons of mustard with 2 tablespoons of water. Stir in the remaining 1/4 cup of cilantro and season the dressing with salt and pepper.
Light a grill or preheat a grill pan. Scrape the marinade off the tuna and season the steaks with salt and pepper. Grill over high heat, turning once, until lightly charred outside and rare within, 2 to 3 minutes per side. Transfer the tuna to a platter and garnish with cilantro. Serve the tuna with the mashed taro, passing the mustard dressing at the table.