Preheat the oven to 400 degrees F. Lightly coat a 9X13-inch pan with cooking spray. Set aside.
In a large pot of lightly salted boiling water, cook the orzo pasta until al dente according to package directions. Drain (but don't rinse) and set aside.
While the pasta is boiling, prepare the cheese mixture. In a 2- or 3-quart saucepan over medium heat, melt the butter. Whisk in the flour until smooth and let the mixture cook for 1-2 minutes, stirring constantly (this helps get rid of the floury taste).
Gradually stir in the milk, whisking constantly, and let it cook at a very gentle simmer for 4-5 minutes until it has thickened considerably.
Off the heat, stir in the cheddar and Monterey Jack cheeses, one handful at a time, making sure each addition has melted before adding the next. Stir in 1/2 cup of the feta cheese (it may not melt all the way - that's ok).
Stir in the oregano, salt and pepper. Add the undrained diced tomatoes and the spinach. Stir the mixture well to allow the spinach to wilt down a bit.
Stir in the cooked orzo. Spread the pasta mixture into the prepared baking dish and sprinkle with remaining 1/4 cup feta cheese. Bake for 15-20 minutes until golden and bubbly.
Let sit for 5-10 minutes before serving.
Notes
Cheese: I normally stay away from reduced fat cheese because of the taste/texture but ended up using a lowfat sharp white cheddar cheese (from Cabot) because it was all I could find and it worked really, really well. The texture might have been slightly different had I used a full fat cheese but we were none the wiser and it saved a few calories.