Preheat oven to 350°F (175°C). Prepare a full sheet tray or large catering tray (approximately 18x26 inches) with 1–1.5 inch sides: lightly grease the tray with 1 tablespoon of the butter or a small amount of oil to prevent sticking.

Sauté aromatics

In a large (6–8 quart) heavy pot or tilt skillet over medium heat, add olive oil and the remaining butter. When butter is melted and foam subsides, add the diced onion and sauté, stirring occasionally, until translucent and just starting to color, about 6–8 minutes. Add the minced garlic and cook 30–60 seconds until fragrant, taking care not to brown the garlic.

Combine rice and liquid

Add the uncooked long-grain rice to the pot with the aromatics and stir to coat the grains with butter/oil. Cook, stirring, for 1–2 minutes to slightly toast the rice for better texture.

Pour in the water or stock and add the kosher salt. Stir to combine and bring the mixture to a gentle simmer over medium-high heat. Once simmering, cover the pot and reduce heat so the liquid maintains a very gentle simmer. Simmer for 8–10 minutes — you want the rice to have absorbed most of the liquid but still be slightly undercooked (par-cooked) so it finishes evenly in the tray.

Finish in full tray and bake

Carefully transfer the par-cooked rice mixture into the prepared full tray and spread it into an even layer using a spatula. If mixture seems dry or the rice hasn’t absorbed enough liquid, sprinkle an additional 1/2–1 cup of hot stock/water across the tray and gently fold to incorporate.

Cover the tray tightly with heavy-duty aluminum foil (or a lid if available) to trap steam. Place tray in the preheated 350°F (175°C) oven and bake for 20–25 minutes, or until the rice is fully tender and the liquid is absorbed.

After baking, remove the tray from the oven and keep covered for 5 minutes to rest and let steam finish cooking the rice.

Finish and garnish

Uncover and use a fork or spatula to gently fluff the rice across the tray, breaking up any clumps and creating an even surface.

Season to taste with freshly ground black pepper, adjust salt if needed, then drizzle the lemon juice and scatter the chopped parsley evenly over the rice. Toss gently to distribute.

Serve directly from the tray or transfer to heated serving pans. Keeps warm covered for up to 1 hour; if holding longer, add a few tablespoons of hot stock and re-cover to retain moisture.