Preheat oven to 400 degrees.

Cook orzo in a pot of boiling water until al dente. Drain and set aside.

In a heavy saucepan over medium heat, melt the butter. Whisk in the flour until smooth and let cook for thirty seconds.

Add 1 1/2 cups of milk, 1/2 a cup at a time, whisking between additions until smooth. Cook for five minutes, whisking frequently. It will thicken up a lot.

Add the cheddar and 3/4 cup feta, and stir until the cheese is melted.

Add salt, pepper and oregano followed by the last 1/2 cup of milk, the diced tomatoes (don't drain them!) and chopped spinach. Stir well (the spinach will wilt down), then add the orzo.

Stir to combine, then pour into a baking dish and top with remaining feta cheese.

Bake for 15 minutes, until cheese is slightly bubbly and brown on top.

Let stand for a few minutes before serving.