Prepare the skillet: Lightly coat a 10-inch cast-iron skillet with olive oil cooking spray. Set aside. (No preheating required.)
Prepare jalapeños: Slice the jalapeño into thin rounds and remove the seeds. (Seeds can burn in the oven and turn black.) Set aside.
Mix dry ingredients: In a large bowl, whisk together cornmeal, flour, sugar, baking powder, baking soda, and salt.
Whisk wet ingredients: In another bowl, whisk milk, Greek yogurt, eggs, and melted butter until smooth.
Make the batter: Stir the wet mixture into the dry ingredients until just combined. Fold in the shredded cheddar cheese.
Bake: Pour the batter into the greased skillet, smoothing the top. Arrange the jalapeño slices evenly across the surface. Bake at 400°F (200°C) for 20–25 minutes, or until golden and a toothpick in the center comes out clean.
Cool and serve: Let the cornbread rest for 10 minutes before slicing. Serve warm with butter or honey.