Preheat the grill to high. Heat ½ tablespoon olive oil in a large non-stick frying pan, then add the onion and garlic, and cook over a high heat until they begin to caramelise.
Add the chopped spinach and tomatoes to the pan, and continue to cook until the spinach begins to wilt. Transfer to a plate to keep warm and carefully wipe out the pan with kitchen towel.
Heat another ½ tablespoon olive oil in the frying pan and add the beaten eggs. Tilt the pan to spread the eggs evenly over the base and cook for 3-4 minutes.
As it cooks, push the edge of the omelette into the centre of the pan, allowing the runny egg to fill its place around the outside. You will end up with cooked egg covering the bottom of the pan.
When the base is golden, pour the spinach mixture on one half and top with grated Tickler mature Cheddar. Place the omelette under the grill until firm, then remove and fold in half. Cut into portions to serve.
Cook's tip: This omelette is surprisingly filling, so try serving with a fresh young leaf salad on the side.