Preheat your oven to 375°F / 190°C. Lightly oil a 2-quart baking dish. 2.
In a large bowl, beat the eggs.
Whisk in the milk (if using), salt, and a generous amount of pepper.
Stir in the cooked rice, shredded Gruyère, and your prepared cooked vegetables until everything is well combined.
Pour the mixture into your prepared baking dish. 3.
Sprinkle the breadcrumbs evenly over the top. For an extra-crispy topping, you can toss the breadcrumbs with the olive oil before sprinkling.
Drizzle any remaining olive oil over the surface. 4.
Bake for 35 to 45 minutes, or until the gratin is sizzling, golden brown on top, and set in the center.
Let it rest for at least 10 minutes before serving to allow it to set properly.
Slice cold leftovers into small triangles or squares to serve as elegant, bite-sized hors d’oeuvres at your next gathering.