Prep the radishes and greens: Separate the radishes from their greens. Wash both thoroughly.
Pat dry. Trim and thinly slice the radishes. Roughly chop the greens, removing any thick or tough stems.
Toast the nuts or seeds: In a dry skillet over medium heat, toast for 3–4 minutes until fragrant.
Let them cool slightly.
Make the radish green pesto: In a food processor, add chopped radish greens, toasted nuts or seeds, Parmesan (or nutritional yeast), garlic, lemon zest, and a pinch of salt and pepper. Pulse to combine. With the motor running, stream in olive oil (start with 1/3 cup and add more as needed) until smooth but still textured.
Taste and adjust with lemon juice, salt, and a tiny drizzle of honey or maple if it tastes too bitter.
Toss the salad base: In a large bowl, add baby greens, sliced radishes, cucumber, and red onion. Add a splash of lemon juice, a small drizzle of olive oil, and a pinch of salt. Toss gently.
Add pesto: Spoon a few tablespoons of radish green pesto over the salad.
Toss to coat lightly. Add more pesto to taste. If it feels thick, loosen with a bit of lemon juice or a teaspoon of water.
Finish and serve: Crack fresh black pepper over the top.
Add extra shaved Parmesan if you like. Serve right away with crusty bread or roasted chicken, or keep it light as a standalone dish.