Place stem pieces in a small pot and fill it with enough water to cover them halfway up. Toss in the garlic and a few pinches of salt. Simmer until the stems become knife-tender (about 20 minutes).
Drain and let cool.
In a food processor, pulse the cooked stems. Add everything else, drizzling in the olive oil at the end. Taste and adjust to your liking.
Notes
Feel free to experiment with other types of stems & herbs. Kale stems are just what I happen to have most often. On this particular day, I also had some parsley and a little bit of basil to use up.
Recipe by Love and Lemons at https://www.loveandlemons.com/kale-stem-pesto/