Preparation

• In a mini food processor or blender, combine the basil, vinegar, mayonnaise, and 2 Tbs. of the oil. Lightly season with salt and process until well blended.
• Put the mushrooms in a large bowl and toss with enough dressing to lightly coat.
• Heat the remaining 2 Tbs. oil and the butter in a large cast-iron skillet over medium-high heat. Season the steaks with salt and pepper, and cook, flipping once, until nicely browned on one side, 4 or 5 minutes. Flip, add the garlic to the pan to flavor the oil, and cook, basting several times with the fat, until rare, another 4 minutes. Transfer to a cutting board.
• Spoon the mushrooms onto a platter or plates. Slice the steaks against the grain, and lay the slices on top of the mushrooms. Top with the onions, and drizzle with the remaining dressing. Garnish with small basil leaves.