Prepare spaghetti in boiling salted water according to package instructions. Drain, quench and allow to drain. In the meantime drain mozzarella. Wash, clean and slice the tomato.
Dice mozzarella roughly. Whisk eggs and season with salt and pepper. Wash the sage, dab dry and, except for something to garnish, pluck off. Heat oil in a medium sized coated ovenproof pan.
Layer spaghetti, tomato and mozzarella. Pour eggs over it. Bake in the preheated oven (electric cooker: 175 °C/ convection oven: 150 °C/ gas: level 2) for about 15 minutes. Take the pasta omelette out of the oven and turn out onto a plate.
Garnish as desired with remaining sage.