Heat a grill to medium-high and brush grates with cooking oil. Place lemon halves, cut sides down, on grill. Close lid and grill until lemon is grill-marked, 1 to 2 minutes. Let cool enough to handle. Squeeze juice from lemon into a small bowl; discard lemon.Prepare dressing: In a small food processor, blend lemon juice, tarragon, 1½ tbsp olive oil, vinegar, mustard, salt and pepper until well combined. Set aside.Brush tomatoes, corn and halloumi all over with remaining 1 tbsp olive oil. Place corn on grill. Close lid and grill, turning often, until bright yellow and grill-marked, 7 to 8 minutes. Transfer to a cutting board and let cool enough to handle.Meanwhile, place halloumi on grill. Close lid and grill, turning once, until grill-marked, about 4 minutes. Place tomatoes, cut side down, on grill. Close lid and grill, without turning, until slightly softened and grill-marked, about 2 minutes.Halve each tomato piece lengthwise to make wedges. Cut kernels from corn. Divide tomatoes and avocado among plates. Top with corn, halloumi, onion and dressing.