Preheat the oven to 360F/180C. And grease the sides and bottom of a 24 inch springform (or quiche form) lightly with melted butter (or olive oil).
Heat up a tablespoon of olive oil in a skillet on medium heat. Once the oil is hot, add the chopped onion and garlic until it is softened.
Add the spinach to wilt it. Then add half of the cherry tomatoes and stir to combine. Sprinkle salt and pepper on it and set it aside to cool off a bit.
When the mixture has cooled off a bit, check if the spinach mixture is not too wet still. If so, squeeze out most of it.
Add the eggs, cheeses, milk and salt & pepper to a mixing bowl and whisk it together.
Open the package of phyllo dough and unroll it gently. Lift the first sheet of the pile, carefully transfer it to the greased springform and drape it over the sides.
Brush the first sheet with melted butter and all following sheets as well.
To make sure the springform is evenly 'loaded', alter the position/angle of each sheet. 4 sheets of phyllo dough are enough.
Add the spinach mixture to the springform and pour the egg-cheese mixture over it. Divide the remaining cherry tomatoes over the pie.
Fold back some of the phyllo sheets to create an artistic edge. Make sure the different sheets are separate from each other to get a crunchy, not too thick crust.
Bake the phyllo pie for 35-40 minutes until the egg mixture has just set. Check the pie after 30 minutes to see if the egg mixture has already set and the crust edges are not too dark already.