Grate the halloumi into a mixing bowl. Finely dice the onion and crush the garlic. Add the onion and garlic to the mixing bowl along with the lemon zest, cauliflower rice, thyme leaves, and black pepper. The halloumi will be quite salty, but you may add a pinch more salt if you wish. Mix well to combine.
Crack the egg into the mixing bowl. Mix everything together well so that it is all evenly coated in the egg. The mixture should be slightly sticking together.
Add the tahini to the bowl. Mix everything together well to combine. The mixture should be holding its shape. Leave to stand for 10 minutes to firm a little.
Lightly oil a non-stick skillet. Heat the skillet over a medium/high heat. Form the halloumi mixture into 4 even-sized patties with your hands. Carefully lower the patties into the hot skillet. Leave to cook for 4-5 minutes on one side or until browned and firm enough to flip. Carefully turn the patties and cook on the second side for 4-5 minutes more. The patties should be golden brown all over and firm. Depending on the size of your skillet, you may need to cook these in two batches. Serve the fritters hot with your preferred Keto sides.