Heat half a tablespoon of olive oil in a non-stick skillet over a low/medium heat. Thinly slice the onion. Add the onion to the skillet and pan-fry gently for a few minutes until tender, lightly golden, and starting to crisp. Remove from the skillet and set aside.
Wipe any excess oil from the skillet and turn the heat up to medium/high. Slice the halloumi lengthwise into two even-sized rectangles. Place the halloumi pieces in the hot, dry skillet. Cook for 2 minutes until browned, then turn and cook on the second side for a further 2-3 minutes until golden all over.
Add the remaining olive oil, tahini, garlic powder, salt and lemon juice to a small bowl. Mix well until smooth and combined. Taste and adjust seasonings as required.
Arrange the lettuce leaves on serving plates. Thinly slice the tomato and avocado. Arrange the tomato and avocado in the lettuce cups.
Place the fried halloumi on top of the avocado. Scatter over the fried onions. Drizzle over the tahini dressing and serve immediately with your preferred Keto sides.