In a pot with boiling salted water, add the spaghetti, stir and boil for about 6-7 minutes. Remove pot from the heat and drain them.
In the meantime in another pot heat 4 tablespoons of the olive oil and remove from heat. Add into the pot the spaghetti, feta, cinnamon, season with salt and pepper, and stir to combine. Then we add 4 of the eggs, lightly beaten, mix well and transfer the mixture into a greased medium sized baking pan.
Preheat the oven at 350 degrees F.
Mix in a bowl 1 1/2 cups of milk with the remaining olive oil and pour it over the spaghetti to cover. We can add, accordingly, some more milk if needed.
In another bowl, beat lightly the 4 remaining eggs, add the kasseri, the remaining 1/2 cup of milk and mix to combine. Pour this mixture over the spaghetti pie and spread it evenly.
Bake in the preheated oven for about 40-45 minutes, until golden brown.
Remove from the oven and let it stand for 10 minutes. Cut into pieces and serve the makaronopita, garnished with arugula leaves.