Preheat your oven to broil or grill setting on medium heat.

Slice tomatoes in half horizontally. Heat oil in an 8-inch non stick pan or skillet over medium-high heat. Once hot, add the garlic, stirring while it cooks for about one minute until fragrant. Add the tomato slices and basil. Continue cooking until the tomatoes are becoming slightly blistered and soft.

While the tomatoes are frying: whisk the eggs, milk and a pinch of salt together in a medium-sized bowl until well combined. When tomatoes are ready, transfer HALF of the tomato mixture onto a warmed plate; cover and set aside.

Pour the egg mixture into the remaining tomatoes in the pan, stirring the tomatoes around through the eggs (for about 20 seconds) to mix the flavours through.

Reduce heat to low-medium; add in the spinach leaves and arrange the mozzarella slices evenly over top, pressing them down slightly into the egg, and cook until the eggs are almost set (the centre will be slightly runny). It takes about 8 minutes (approximately).

Sprinkle the extra cheese (if using) over the top and transfer pan/skillet into the preheated oven. Grill/broil until the top is golden, puffed up and cooked through. The edges and the bottom of the frittata will be golden.

To serve, warm the remaining tomatoes and basil mixture, and spoon over the top. Drizzle with the balsamic glaze.