Slice the Baby Gem lettuces into quarters just down the root of the lettuce so it stays together.
Heat a dry frying pan or griddle pan to a medium heat and place the lettuce quarters on it. Leave for 1-2 mins until the lettuce starts to brown slightly (you may need to do this in 2 batches).
Make the dressing by putting all the ingredients in a jug and whisking until smooth. Slice the feta into 1cm blocks. Divide the lettuce between 2 plates, top with grapefruit segments and feta, then drizzle the dressing over the salad. Sprinkle over the lemon zest to garnish.