For the Rigatoni:
In a large bowl, mix together the flour and water with your hands, or pulse them in a food processor until the mixture looks like slightly damp sand. You don’t want this to be too wet or it will stick to the machine and clog it up. It should be drier than you might expect: when you squeeze a small amount in your fist, it should hold together, though it will feel and look far drier than a typical egg pasta dough. Bring the dough together into a ball and wrap in plastic. Let it rest at room temperature for 30 minutes.
In the meantime, fit your pasta extruder with the rigatoni plate, and attach it to your stand mixer. Sprinkle a heavy dusting of semolina onto a large sheet pan or two so you can arrange the rigatoni in a single layer. Extrude the pasta, cutting it every 2 inches. If you aren’t using a stand mixer, follow the manufacturer’s instructions for your particular pasta machine. Allow the pasta to sit out to dry for 3 to 4 hours. You can freeze it now, if you’d like, on the sheet pan; once firm, transfer to a zip-top bag and return to the freezer for up to a few weeks.
For the Bechamel:
In a medium saucepan, melt the butter over medium heat, then add the flour and whisk together until incorporated. Cook for a minute until you have a blond roux. Steadily stream in the milk while whisking constantly. Increase the heat to medium-high and bring to a simmer. Cook, whisking, for 3 to 4 minutes, until the flour is cooked and you can no longer taste it. Reduce the heat to low and add the Fontina Cheese. Whisk until the sauce is smooth. Season with salt to taste. Keep warm in a double boiler.
Prep the Garnish:
In a small saute pan, heat the olive oil over medium heat until it begins to shimmer. Add the sage leaves and fry until crisp, 1 to 2 minutes. Remove and drain the leaves on a paper towel. When the sage has cooled to room temperature, crumble it into a bowl and gently toss with the walnuts, cheese, and fried shallots. Season with salt and pepper.
Meanwhile, bring a large pot of water to a boil to cook the pasta. Salt the water generously. Lower the heat so the water is at a rolling simmer. Add the pasta to the boiling water and cook until al dente, 3 to 4 minutes. Drain, reserving some of the cooking water.
Mix the drained pasta with the bechamel. Use a ladle of pasta water to thin out the sauce, if necessary. You want to immerse the pasta in a generous amount of sauce, more than just a coating. Divide the pasta and sauce among the bowls. Sprinkle equal amounts of the walnut garnish over each portion.