Toast your shelled walnuts in a 10-inch pan on the stove for about five minutes over medium-high heat, stirring occasionally. The walnuts are ready when they are fragrant. Don't let them burn. Set the pan aside on a cool spot on the stove.
Grate your Parmesan and Romano cheeses, if needed. (I use my blender.)
Place a pasta pot filled 2/3 high with hot water on the stove over high heat. Add about a couple teaspoons of salt to the water and cover the pot.
While the water is heating, prepare your walnut sauce. Place the toasted walnuts and all the remaining sauce ingredients in your food processor or blender. Process until the sauce becomes a smooth paste, scraping down the sides as needed.
When the water is boiling, add your linguine and stir it in. Lower the heat a bit and continue cooking the pasta until it's al dente.
While the pasta is cooking, scoop out a cup of the pasta water (I use my ladle and take two scoops) and place it in a pasta bowl near the stove.
Drain the pasta when it's cooked, setting it in a colander in the sink. Add your butter to the pasta pot, then return the linguine to the pot. Stir it a bit, so it doesn't stick.
Add all of the walnut sauce to the pot, along with one ladle of the reserved pasta water. Stir to combine with the pasta and add extra reserved water as needed, so that the pasta is not dry.
Serve immediately with red pepper flakes and extra Parmesan. If you have any leftovers, stir in any remaining, reserved pasta water to the leftovers and store, covered, in the refrigerator.
The leftover pasta will be dry, so you will need to add some butter or olive oil when reheating it.
It's best to enjoy this dish immediately, since the leftovers will become dry. If you know you'll only need half a pound of pasta, use half of the sauce and store the leftover sauce in a covered container in the refrigerator. Then, you can cook fresh pasta to go with it the next time, adding butter and some reserved pasta water.