2-1/2 quart capacity (10 x 2-inch) round or oval baking dish, or oven-proof skillet
1. TRIM the broccoli rabe. Fill a pot with enough water to cover the greens, add salt and bring to a rolling boil. Blanch the greens for no more than ten seconds; drain at once and plunge into ice water to arrest cooking. Drain again and squeeze out excess water. (For a mellower flavor and tender texture, cook the greens for 2-4 minutes, depending on your “kick” and “crunch” preference.) Chop and set it aside.
2. REMOVE corn from cob. If using frozen corn, pat it dry with a paper towel to remove excess liquid.
3. PREHEAT the oven to 400°F. Arrange a rack on the lower third position of the oven, removing other racks. Grease the bottom and sides of the baking dish or the individual ramekins with butter. Scatter the Parmigiano-Reggiano or Grana Padano in the pan and shake to coat the bottom and sides as much as possible. Don’t be concerned about gaps in coverage. Tap out any excess.
4. MELT half of the butter in a medium-sized, thick-bottomed saucepan. Stir in the garlic. Immediately add the chopped broccoli rabe, corn and minced hot pepper and cook over medium-low heat to marry the flavors, stirring occasionally, 3-4 minutes. Transfer it to a strainer to cool.
5. WARM the milk and half-and-half together in a small saucepan with a heavy bottom, until they begin to simmer; don’t allow it to break into a boil. Keep it warm. (Alternatively, measure out the milk into a glass measuring cup and heat it in a microwave.)
6. MELT the remaining butter in another, medium-sized heavy saucepan. Over low heat, add the flour and stir with a wooden spoon or whisk to prevent lumps from forming. Stir constantly, about 2 minutes. Do not let the mixture brown.
7. REMOVE the pan from the heat and add the warm milk and half-and-half mix, one trickle at a time, until you have used up 1/2 cup, stirring constantly to blend. Return the pan to the burner over low heat and stir in the remaining milk, still at a trickle, slowly and gradually. If lumps start to appear, you are probably adding the milk too quickly. Should this happen, turn off the heat and stir, pressing the lumps against the side of the pan and continue to blend in the hot milk very gradually. When all the warm milk has been added, simmer the sauce over low heat for another 5 minutes or so, until it is very thick. Stir in 1/4 teaspoon of salt.
8. REMOVE the sauce from the stove and transfer it to large mixing bowl. Whisk in one egg yolk at a time. It should be tepid to warm when folding in the remaining ingredients.
9. BEAT the egg whites in a bowl with a pinch of salt on medium speed until they are frothy, about 1 minute in a stand mixer, up to 2 minutes using a hand-held mixer. Add the cream of tartar and continue to beat until soft peaks form, 1 to 2 minutes. Whisk a quarter of the whites into the sauce. Using a rubber spatula, gently fold in the rest in two batches, working carefully to combine the ingredients without deflating the whites. At last, fold in the cooled broccoli rabe, corn, and hot pepper mixture along with the Cheddar or Gruyère.
10. TRANSFER the batter into the prepared baking dish, using the rubber spatula to spread it. (At this point it can be covered with an upturned pot and set aside for up to an hour before baking.) Slide it onto the prepared oven rack. Bake until a puffy, golden brown crust is formed on the surface and a skewer inserted into the middle comes out clean, about 35 minutes. Serve at once. It must be piping hot.