In a medium bowl, whisk together the lemon juice, honey or maple syrup, poppy seeds, Dijon mustard, and kosher salt. Then gradually whisk in the olive oil, 1 tablespoon at a time, until a creamy emulsion forms.
Wash the salad greens. Wash the grapes and cut them in half.
In a dry skillet, toast the walnuts over low heat, stirring frequently, until browned and fragrant, about 5 minutes.
To serve, place greens on a plate or bowl; top with grapes and toasted walnuts. Drizzle with poppyseed dressing. Any extra dressing stores in the refrigerator for up to 2 weeks.