Combine olive oil, minced garlic, thyme, salt and pepper in a small bowl. Combine the corn, onion and tomatoes in a large bowl. Place the steak in a shallow pan. Toss the vegetables with half of the marinade. Pour the remaining marinade over the steak, turning to coat both sides. Refrigerate for 30 minutes or until ready to grill. Meanwhile combine the sour cream, mustard, vinegar, red pepper flakes, and milk in a small bowl. Refrigerate the dressing. Heat the grill. Grill the steak, covered, over medium heat for 4 to 5 minutes for medium-rare, turning once. Cover loosely with foil and let stand for 5 minutes before slicing. Cook the vegetables, covered, over medium heat for 1 to 4 minutes, turning and removing the vegetables when crisp-tender and lightly charred. Arrange the romaine lettuce on platter or individual plates; top with steak, vegetables, avocado and blue cheese. Drizzle with the dressing before serving. * Tip: Place toothpick through onion wedges to keep onion together during grilling; remove before serving.