Shrimp Cakes with Scallion Dip

1/2 cup mayonnaise

1/2 cup chopped fresh parsley

1 scallion, roughly chopped

1 small clove garlic, grated

2 tablespoons fresh lemon juice

Kosher salt and freshly ground pepper

1 pound medium shrimp, peeled and deveined

1 cup panko breadcrumbs

1 large egg, lightly beaten

1/2 teaspoon grated lemon zest

3 tablespoons vegetable oil, plus more for forming

1 5-ounce package mixed greens (about 6 cups)

4 radishes, thinly sliced