How to Make It
Preheat the oven to 425°. Cut 1/2 inch off the top of the garlic head and set the head on a piece of foil. Drizzle the garlic with olive oil, wrap tightly and roast until tender, about 45 minutes. Unwrap the garlic and let cool slightly, then squeeze the cloves into a small bowl. Mash with a fork until smooth. Whisk in 1/4 cup of the olive oil and the vinegar and season the dressing with sea salt and pepper.
Meanwhile, in a large nonstick skillet, heat 1/4 cup of the olive oil. Add the bread and cook over moderate heat, tossing, until golden and crisp all over, about 5 minutes. Arrange the croutons on plates.
Using a sharp knife, make a 1/4-inch-deep cut down the length of the filet mignon. Turning the filet and rolling it out as you go, spiral-cut the meat until you have a long, rectangular piece that's about 1/4 inch thick; alternatively, have your butcher butterfly the steak for you. Make 1/2-inch-deep slits every 2 inches along the grain all over the steak. Rub the steak with 1 tablespoon of the olive oil. Season with sea salt and pepper and arrange the chives on top, tucking them into the slits.
Preheat a cast-iron skillet. Add the remaining 1 tablespoon of olive oil to the skillet and cook the steak, chive side up, over moderate heat until browned, about 3 minutes. Flip the steak and cook until the chives are charred and the steak is medium-rare inside, 2 to 3 minutes longer. Transfer the steak to a cutting board and let rest for 5 minutes.
Top the croutons with the basil and microgreens and spoon the roasted garlic dressing on the greens. Slice the steak against the grain and serve alongside.