Place your tomato top side down and slice 5 even strips, leaving enough room at the bottom to keep the tomato intact. Place a basil leaf down each slot and crumble the feta across and either side of the tomato.
For the sweet balsamic glaze, pop your balsamic vinegar and brown sugar in a pot over medium heat and stir until the sugar dissolves. Bring to the boil and then reduce heat to low and stir occasionally until the glaze thickens and could coat the back over a spoon. Should take around 20 minutes on a simmer. Be careful the glaze doesn't overcook and the sugar burns, giving off a real bitter taste.
Drizzle over some olive oil and splash your balsamic glaze with a spoon (measure to preference).