Bring a large pot of generously salted water to a boil. Blanch the trimmed green beans for 4–5 minutes, until bright green and just tender, then drain and immediately transfer them to ice water. Drain again very thoroughly.

Heat a large skillet over medium heat and cook the diced cured chorizo for 3–4 minutes. Once it releases its oil and begins to brown, remove excess fat if necessary.

Add the chopped onion and sliced mushrooms to the skillet. Cook for 6–8 minutes, until softened and most of the mushroom moisture has evaporated, then add the garlic and cook for 30 seconds.

Stir in the butter or olive oil, then sprinkle the flour over the vegetables and chorizo. Cook for about 1 minute, stirring, so the flour loses its raw taste.

Gradually whisk in the stock and heavy cream, scraping the skillet as you stir. Simmer for 3–5 minutes until the sauce thickens, then season with the kosher salt and black pepper.

Stir half of the shredded cheese into the hot sauce until incorporated. Fold in the drained green beans and cooked chorizo, taking care not to break the beans.

Transfer the mixture to a greased 9-by-13-inch baking dish and spread it into an even layer. Sprinkle the remaining cheese over the top.

Bake uncovered at 375°F (190°C) for 20 minutes. Scatter the crispy fried onions over the casserole and bake for another 5–10 minutes, until bubbling and golden.