Step 1Bring large pot of water to a boil over high heat. Add salt and spaghetti and stir for a good minute so spaghetti strands don't stick together. After 12 minutes, fish a strand of spaghetti out with a fork and run it under cold water. If spaghetti is tender and not firm or hard, it is ready. (If you haven't finished preparing sauce, just turn heat off under pot of spaghetti and let it stay hot until sauce is finished.)
Step 2As spaghetti cooks, cut chile into 1/4-inch strips. Hold strips in bunched up pile and chop crosswise into 1/8-inch pieces. Set aside. Cut stems from tomatillos and slice tomatillos in 1/4 inch slices. Hold each sliced tomatillo together and slice into small pieces crosswise.
Step 3Place garlic, cilantro, onion, pepper and tomatillos in small saucepan and set aside. Just before draining spaghetti, warm green sauce 3 to 5 minutes over medium heat; this isn't to be cooked, just barely warmed so spaghetti stays hot.
Step 4Drain spaghetti into colander and shake colander to remove as much excess water as possible. Combine spaghetti and warmed sauce in mixing bowl and serve. Garnish with Parmesan cheese.