Broccolini, or baby broccoli, is typically sold in bunches. I can eat an entire bunch on my own, so I always make at least two bunches so others can have some, too. The sauce is plenty to top two bunches, but it holds well in the refrigerator, so a double batch is always a good thing.
Fill a wide saute pan with water to an in or two from the top (enough space to make room for the broccolini). Add the 2 tablespoons kosher salt and bring the water to a boil. Lay the broccolini in the boiling water, using tongs to submerge as much of the broccolini as possible.
Continue to boil the water for two minutes, then turn off the heat and allow the broccolini to cook in the hot water for another 12-14 minutes for al dente.
Meanwhile, toast the pine nuts in a dry saute pan over medium low heat, stirring or shaking the pan occasionally, until they are light golden brown.
Transfer all but two tablespoons of the pine nuts to a food processor (prep size works fine for a single recipe of the sauce). Add the garlic and salt, and puree, stopping a couple of times to scrape the sides of the bowl. Add the lemon juice and then the water, stopping to scrape down the bowl as you go as you puree.
With the processor running, slowly drizzle in the olive oil until the sauce is smooth and fairly loose. Taste and add more water or lemon juice if needed.
Remove the cooked broccolini from the pan (or pour off the water). Toss the broccolini with a couple of tablespoons of olive oil, arrange on the serving platter (if using), and top with a few grinds of black pepper.
Spoon the sauce across the center of the broccolini stems, sprinkle with the remaining two tablespoons of toasted pine nuts, and finish with a dusting of sumac, if using.