Preheat your oven to 375°F and lightly grease a 3-quart oval baking dish.

Bring a large pot of salted water to a boil. Add the trimmed green beans and blanch them for 3–4 minutes, just until they turn bright green and slightly tender. Drain and immediately transfer to an ice bath to stop the cooking. Drain again and set aside.

In a large skillet over medium heat, melt the butter. Whisk in the flour to make a roux and cook for about 1 minute, stirring constantly, until smooth and lightly golden.

Slowly pour in the milk, heavy cream, and chicken stock, whisking continuously until the mixture is smooth and begins to thicken.

Add in the cream of mushroom soup, Boursin cheese, Worcestershire sauce, complete seasoning, and shredded Mexican cheese. Stir until everything is melted and combined into a creamy sauce. Taste and adjust seasoning as needed.

Add the blanched green beans and pancetta or crumbled bacon to the sauce. Gently stir to coat all the beans evenly.

Transfer the mixture to your prepared baking dish and spread it out evenly. Cover the dish loosely with foil.

Bake for 40 minutes. Remove the foil, sprinkle the crispy fried onions over the top, and bake uncovered for another 5 minutes, or until the topping is golden and crisp.

Let cool slightly before serving.