This preparation is classic for a reason. Obviously, spinach is the usual green recruited for this dish, but I love using tender kale in its place. You can make in advance, then simply warm with the breadcrumbs and top with lemon juice just before serving. This should have a baked feeling, so the oven time is important to achieve the classic "steakhouse side dish" effect.
Heat 1 tablespoon oil in a large braiser over high heat. When the oil begins to smoke, add half the kale to the pan. Season with salt and cook, stirring, until the leaves take on a deep green hue and lose 50 to 75 percent of their volume, 2 to 3 minutes. Transfer the kale to a large bowl with the spinach. Repeat with the remaining oil and kale, adding it to the bowl as done.
Return the braiser to high heat and add the cream. Simmer the cream until it reduces to about 1 3/4 cups, about 5 minutes. Add the cream to the greens and stir to combine. Add the Parmesan, Gruyere, Worcestershire, chile flakes, nutmeg, lemon zest and juice and salt to taste. Return the greens to the braiser and top with the panko and a drizzle of oil. Bake until warmed, 12 to 15 minutes.