Chop the sweet potatoes in small cubes about ½ inch cubes. Place in bowl and drizzle 1 ½ tablespoon olive oil, mix until all pieces are coated with the oil. Spread in a pan and roast for 20-25 minutes. Remove and set aside, lower oven temperature to 350 F (180 C).
In the meantime sauté the onion in 2 teaspoons of olive oil until translucent.
In a large bowl mix the crumbled feta, eggs, milk, 1 tablespoon herbs de provence , 1 teaspoon salt, freshly ground pepper, 1 ½ tablespoon olive oil. Add the onion (let it cool a few minutes before adding) and the sweet potato cubes. Mix gently and set aside.
Brush a 10 inch tart pan with olive oil. Place 6 sheets of phyllo in the pan, brushing each layer with olive oil. Cut off the leftover pieces hanging over the sides of the pan. * If you are using homemade phyllo see notes.
Empty the sweet potato mixture in the pan, spreading it evenly with a spatula. Sprinkle with parmesan.
Cover the tart loosely with aluminum foil and bake for about 20 minutes. Remove aluminum foil and bake for another 20-25 minutes.
Remove from oven and let it cool before slicing.
Notes
If you are using homemade phyllo, you only place one layer of dough on the bottom and brush with olive oil. You will need to bake 10-15 minutes extra.