Prep and cook the orzo
Bring a pot of salted water to a boil. Heat the broiler to high, and position an oven rack 7 to 8 inches from the heat. While the water heats:
Zest the lemon and juice half.
Strip the mint leaves from the stems; coarsely chop the leaves.
Add the orzo to the boiling water and cook until just tender, 6 to 8 minutes. Drain, reserving 1 cup pasta cooking water.
In a bowl, combine the cooked orzo, lemon zest and juice, and half the mint. Drizzle with 1 to 2 tablespoons oil, season to taste with salt, and toss to combine. While the orzo cooks, prepare the gratin ingredients.