Set a colander in the sink. Bring 3 cups of water to a boil, using either a kettle or a saucepan.
While the water is heating up, cut the onion into a large dice. Place the diced onion in the colander and slowly pour the boiling water on top.
Add the vinegar, salt and sugar to a medium-sized container or jar with a tight-fitting lid, and stir to dissolve. Add the onions, close the lid, and allow to sit for a minimum of 30 minutes.
Prepare the pasta:
Bring a large pot of salted water to a rolling boil, and cook the pasta until al dente (according to package instructions). Before draining, use a liquid measuring cup to carefully reserve 1/2 cup of the pasta water.
In a large bowl, toss together the cooked pasta, ramp pesto, 1/4 cup of the reserved pasta water, lemon juice, and salt.
Divide between plates and top each serving with a generous portion of parsley, crushed red pepper, grated parmesan cheese, and pickled onions.