In the work bowl of a food processor, pulse together nuts, cheese, and garlic until mixture resembles coarse crumbs. Add watercress and basil, pulsing to combine. With food processor running, slowly add olive oil to bring pesto together, scraping down sides of work bowl as needed. Add salt, pulsing to combine.
Notes
Make-Ahead Tip: Pesto can be made up to 3 days in advance, covered tightly, and refrigerated until needed.
Recipe by TeaTime Magazine at https://teatimemagazine.com/watercress-pesto-steak-crostini/