Thinly slice the onion and celery, then finely chop the garlic.
Cut the halloumi into small cubes.
Heat half of the oil in a frying pan, then fry the halloumi for a few minutes until it's brown all over. Transfer to a plate.
Wipe out the pan, if needed, then heat the remaining oil and gently sauté the onion and celery over low heat for about 5 minutes or until soft.
Add the garlic and oregano and stir for a minute. Add the drained lentils, mustard and lemon. Stir to combine. Season to taste. Add the spinach and a splash of water. Put a lid on and steam for 3-5 minutes until the spinach has wilted.
Stir through the halloumi and serve.