The tomatoes give this dish some flare and the spinach adds color and nutrition. Posted here to be easy to find from Cover & Bake by America's Test Kitchen.
For the topping: process bread, pine nuts and butter in a food processor until coarsely ground (about 10 1 second pulses). transfer to a bowl & mix in Parmesan cheese.
For the filling: Preheat oven to 400*F.
In a large heavy bottom pot (dutch oven), heat oil & garlic over medium high heat until garlic is lightly golden, stirring often, about 2 minutes.
Add the spinach in handfuls and stir until just wilted, about 4 minutes. transfer spinach to a colander set in the sink and gently squeeze out excess moisture. Coarsely chop the spinach & set aside.
Wipe the pot clean and melt the butter over medium heat. Add the flour, stirring constantly, cook until golden, about 1 minute.
Slowly whisk in the broth and milk, bring to a simmer, whisking often.
Add the rice, oregano, salt, and pepper. Return to a simmer then reduce heat to medium low.
Cover and cook, stirring often, until rice has absorbed most of the liquid & is just tender, about 25 minutes.
Add the chicken and cook uncovered, stirring occasionally, until it is no longer pink, about 5 minutes.
Off heat, stir in the sun-dried tomatoes, mozzarella, and spinach.
Pour into a 9x13" baking dish and sprinkle with bread crumb topping.
Bake until the topping is browned and the casserole is bubbling, 20-25 minutes.