Preheat a grill pan or outdoor grill to medium heat.
Brush the halloumi and avocado slices with olive oil. Place the avocado and halloumi on the grill until char marks appear, about 2 to 3 minutes each side.
Meanwhile make the dressing by combining all the ingredients in a small food processor and blending until smooth. Add milk only if needed for consistency.
In a medium skillet, melt the butter over medium heat. Add the panko to the butter and cook, tossing often until browned, about 4-5 minutes.
Place the arugula on a large platter. Add the tomatoes and onions. Sprinkle with the panko. Layer on the avocado and halloumi then drizzle with dressing.