Preheat oven to 180°C. Whisk together the eggs, cheddar and mustard in a large bowl. Season with salt and pepper.
Heat the oil in a 16cm-diameter (base measurement), 1.25L (5-cup) capacity ovenproof frying pan over medium-high heat. Add the bacon and onion and cook, stirring occasionally, for 5 minutes or until light golden.
Add the spinach to the pan and cook, tossing, for 1-2 minutes or until the spinach wilts.
Reduce heat to very low. Pour the egg mixture into the pan. Shake to evenly distribute the egg. Arrange the tomato on top. Cook for 5-8 minutes or until the frittata is set around the edge but still runny in the centre. Place the frying pan in oven and bake for 30 minutes or until the frittata is golden brown and just set.
Sprinkle the parsley over the frittata. Cut into wedges to serve.