How to Make It
For the tuna, sprinkle each steak with 1/4 teaspoon each of the salt and pepper and 3/4 teaspoon of the thyme and rub the seasonings into the tuna. Turn the steaks over and repeat on the other side.
Put 1 teaspoon of the oil in each of two 10-inch nonstick skillets and place over high heat until just smoking. Add 1 tuna steak to each skillet and sear until browned on the outside and rare in the center, about 1 minute per side. Let cool. (This can be done up to 8 hours in advance).
For the vinaigrette, combine all of the ingredients in a bowl, except for the oils. Whisking constantly, very slowly drizzle in the olive oil and then the corn oil. Store, tightly covered, in the refrigerator for up to 1 week.
Using a very sharp, thin-bladed knife, slice the tuna crosswise 1/8-inch thick. (The recipe can be made to this point up to 3 hours ahead; cover the tuna with parchment paper and refrigerate.)
To serve, toss the salad greens, bean sprouts, mint, coriander and peanuts with half of the vinaigrette. Make a tall mound of salad in the center of 4 dinner plates. Drape the tuna slices over the salads in a circle, with one end of each slice meeting at the highest point of the salad. Drizzle the remaining vinaigrette around (but not touching) the tuna and serve.