Preheat oven to 375°F (190°C). Grease a baking dish with nonstick spray or a little olive oil.
In a skillet over medium heat, warm olive oil. Add onion and sauté until translucent.
Stir in garlic and spinach. Cook until spinach is wilted, about 3-4 minutes. Remove from heat.
In a large bowl, whisk eggs and milk together. Add cooked rice, spinach mixture, feta, dill, salt, and pepper. Mix until well combined.
Pour mixture into prepared baking dish and spread evenly.
Bake for 30–35 minutes, or until the top is golden and the casserole is set.
Let cool for a few minutes before serving. Enjoy!