Preheat oven to 400 degrees.
Place the sliced bread on a rimmed baking sheet and coat generously with cooking spray.
Bake the bread until toasted and light golden brown, about 5 minutes. Set aside.
In a medium bowl, combine the sliced radishes, olive oil, lemon juice, and salt and pepper. Lightly toss to thoroughly coat the radishes.
To assemble the crostini: Gently spread about 1-2 teaspoons (however much you like) cheese on top of the bread.
Add a thin layer of the mixed greens and top with the radish slices. Serve immediately.