Preheat the oven to 400ºF. Coat a 9 x 13 casserole dish with olive oil or cooking spray. Set aside.

In a large skillet, heat 2 tablespoons of butter and a teaspoon of olive oil over medium low heat until the butter melts. Add the onions and leeks and stir to coat in the butter mixture. Turn the heat to low and allow to cook for one hour, stirring every ten minutes, until golden brown in color. The vegetables should just barely sizzle as they cook. If they are browning too quickly lower the heat as much as possible and if they don't seem like they're cooking at all raise the heat by small increments until they begin to do so. Once they are done remove from the heat and set aside.

Meanwhile, cut the top quarter off the head of garlic. Drizzle the other teaspoon of olive oil over the top and wrap it tightly in aluminum foil. Place in the oven and allow it to roast for 30 minutes. Remove and allow it to cool before squeezing the garlic out of the papery peel.

Bring a large pot of water to a boil and cook the rigatoni according to the package's directions. Once done cooking, drain, drizzle with olive oil, and shake the colander to keep the noodles from sticking together.

In a small skillet, heat one tablespoon of olive oil over medium heat. Add the spinach and sauté until wilted. Remove from the heat and set aside.

Preheat the oven to broil on high. Heat the remaining tablespoon of butter in a medium saucepan over medium low heat until melted. Whisk in flour until fully incorporated. Whisk in the milk and cream and continue to heat until hot but not boiling. Add the oregano and rosemary and mix well. Add 1 cup asiago and crumble in the feta cheese. Stir until the feta is just beginning to melt.

Remove the pan from the heat and combine the cheese mixture with the rigatoni, spinach, sun-dried tomatoes, onions, leeks, and roasted garlic in the prepared baking dish. Sprinkle with black pepper and remaining Asiago cheese. Place in the oven and broil until lightly browned, about 5 minutes. Remove and serve.