For vinaigrette, pulse onion, vinegar, honey, mustard and tarragon in a food processor or with a hand blender until smooth. While blending, slowly pour in oil until incorporated. If vinaigrette is too thick, whisk in 2 to 3 tbsp (30-45 mL) warm water. Season to taste.
To build salad, grill apricot halves over high heat turning once, just to soften and add grill marks, about 4 minutes. Slice apricot halves into half again and toss with 2 tbsp (30 mL) of vinaigrette. Arrange salad greens on a platter and arrange apricots on top. Sprinkle salad with walnut pieces and Feta and dress immediately before serving (or serve vinaigrette on the side).
Stir vinegar, soy sauce and mustard in a large flat dish. Stir in garlic and onion and pull rosemary leaves off their stem and add. Whisk in oil. Add flank steak and spoon some of the marinade (including garlic & onions over top). Cover dish and chill for 30 minutes up to 12 hours before grilling, turning steak once or twice.
To cook, grill steak over high heat, about 5 minutes on each side (do not cook past medium). Let steak rest on a cutting board for 3 minutes before slicing on the bias (against the grain). Arrange steak on salad or on side plate and sprinkle with salt and pepper before serving.