Heat oven to 375°F. Lightly oil a 9x13-inch baking dish.
Warm olive oil in a large skillet over medium heat. Soften onion for 5 minutes.
Stir in garlic and cook 30 seconds. Add spinach and cook until wilted; remove from heat.
In a big bowl, combine orzo, spinach mixture, sun-dried tomatoes, olives, dill, oregano, lemon zest, and feta.
Whisk eggs, yogurt, milk, lemon juice, salt, black pepper, and red pepper flakes until smooth.
Pour the egg mixture over the orzo blend. Fold until evenly coated; taste and adjust seasoning.
Spread into the prepared dish and smooth the top.
Toss breadcrumbs with Parmesan and olive oil. Scatter the mixture evenly over the casserole.
Bake until set and golden on top, 25–30 minutes. Rest 10 minutes before slicing.
Finish with extra dill and a squeeze of lemon, if you like, and serve warm.