In a skillet over medium-high heat, add halloumi and cook 2 to 3 minutes on each side till golden brown.
In a large bowl, add spinach, cucumber, tomatoes, onion, olives, and chicken and toss together well.
In a bowl, whisk together oil, vinegar, mustard, oregano, salt, and pepper. Set aside dressing.
Serve salad with halloumi and dressing.
Note: there will be leftover dressing. Store in an airtight container in the refrigerator for up to 2 weeks. Bring to room temperature before using.