Cut halloumi cheese into 5 mm thick slices. Heat olive oil in a pan over medium-high heat. Fry halloumi slices 1-2 minutes per side until light brown and crispy.
Cut tomatoes, cucumber, and feta cheese into small cubes. Cut onion into rings. In a bowl, combine tomatoes, cucumber, feta, and onion. Drizzle with olive oil and season with salt and pepper to taste.