Pour the olive oil into the small pan, add the garlic and sautée for about two minutes, then add the rapini with a splash of water and cover.
Cook until the thickest past of the rapini is cooked, but still quite firm, about 5 minutes (remember, it should be a little more undercooked as it still has to go into the oven). Set aside.
Put water into the pastaiola (at least ¾ full) and bring the water to a boil, but while it's heating, begin to make the white sauce.
In a medium pot over medium heat, melt the butter and add the flour, constantly stirring. Keep stirring and do not allow to brown, but cook for about 4 to 5 minutes (or else the flour will taste raw in the sauce).
Add the milk a little at a time, incorporating it into the flour mixture each time until a sauce begins to form. For more detailed instructions click HERE. Add the grated cheese.
Preheat oven to 400°F (200°C):
When the water in the pastaiola comes to a boil, add the 2 tsp of salt and then add the pasta.
Stir the pasta often so that it doesn't stick together. Cook until very al dente (9 minutes for this De Cecco pasta). Drain pasta. DO NOT throw away the water from the pasta yet.
Put the pasta in a large bowl, add most of the besciamella sauce, 6 oz (175 ml) of the pasta water and the rapini and stir to combine.
Put in oven-proof dish, top with the remaining besciamella sauce and place in the preheated oven (a little above the center of the oven). Bake 20-30 minutes or until the top begins to brown.
Serve hot.