Prep and preheat: Preheat the oven to 200°C (400°F). Finely dice the onion and mince the garlic. Grate the cheeses and measure out butter, flour and milk. Having everything ready lets you move quickly when the pasta and rice are done so textures remain right.

Cook the rice: Rinse the 120 g rice under cold water until the rinse runs clear to remove excess surface starch (this helps the rice separate and crisp later). In a small saucepan combine rinsed rice and 240 ml water with 1/2 tsp salt, bring to a boil, then lower to a gentle simmer, cover and cook 12–14 minutes until liquid is absorbed. Remove from heat and let sit, covered, 5 minutes to steam. Fluff with a fork. Cooking rice separately and letting it steam keeps grains intact so they can form a crisp crust when later sautéed.

Caramelize the onions: While rice cooks, warm 1 tbsp olive oil and 20 g butter in a large sauté pan over medium-low heat. Add the diced onion with a pinch of salt and sweat for 5 minutes, then increase heat to medium-low and cook, stirring every 3–5 minutes, until deeply golden and sweet, 20–25 minutes total. Add half the minced garlic in the last 2 minutes. Slow caramelization draws out sugars and concentrates flavor—this is the sweet counterpoint to the cheesy pasta.

Boil the pasta: Bring a large pot of salted water to a vigorous boil (about 1.5 tsp salt per 2 liters). Add 300 g pasta and cook 1–2 minutes less than package directions for al dente — the pasta will finish in the oven. Reserving a saucing time and slightly undercooking prevents a gluey texture after baking.

Make the cheese sauce (roux method): Meanwhile, melt 40 g butter in a medium saucepan over medium heat. Add 30 g flour and whisk 1 minute to cook the raw flour taste and form a roux—this will thicken the sauce. Slowly whisk in 350 ml whole milk in a steady stream to avoid lumps, bring to a gentle simmer and cook 3–4 minutes until the sauce coats the back of a spoon. Remove from heat and whisk in 150 g grated cheddar and 25 g Parmesan until melted and smooth. Season with 1/2 tsp salt, 1/2 tsp black pepper and 1 tsp dried oregano. A warm, properly cooked béchamel gives the pasta a silky, stable creaminess that holds through baking.

Combine pasta and sauce: Drain the pasta, reserving 60 ml (1/4 cup) of the cooking water. Return pasta to the pot, pour in the cheese sauce and toss to coat. If the sauce seems thick, add 1–2 tbsp reserved pasta water to loosen. Fold in half of the caramelized onions (keep rest for topping) and the remaining minced garlic. Tossing while hot helps the sauce cling to the pasta.

Prepare the rice crust: In the same sauté pan used for onions, heat the remaining 1 tbsp olive oil and 20 g butter over medium-high. Add the fluffed cooked rice and spread into an even layer. Let the rice sit undisturbed for 3–4 minutes to begin crisping, then stir and press down again for another 3–4 minutes until golden and nutty in places. Off heat, fold in 15 g grated Parmesan and season with a pinch of salt and pepper. Crisping the rice develops Maillard flavors and a crunchy contrast to the creamy pasta.

Assemble in a baking dish: Lightly butter a 9x13-inch (23x33 cm) baking dish. Spread the warm herbed garlic rice in an even layer on the bottom — this forms a flavorful base and keeps the underside from becoming soggy. Spoon the cheesy pasta over the rice and smooth the top. Scatter the remaining caramelized onions on top, then sprinkle the final 25 g grated cheddar and 20 g Parmesan evenly. For a decorative, extra-crispy finish, press some of the crisp rice pieces onto the top layer.

Bake: Place the dish in the preheated oven and bake 18–22 minutes until bubbling and the top is golden. If you want a deeply golden top, run under the broiler 1–2 minutes while watching closely. Baking melds the flavors and gently finishes the pasta without drying it.

Rest and serve: Let the casserole rest 8–10 minutes before scooping — this lets the sauce settle and makes neat portions. Garnish with chopped fresh parsley if using. Resting ensures clean slices and avoids the sauce running everywhere.