Preheat your oven to 375°F and grease a 9-inch baking dish.
In a skillet over medium heat, melt the butter and sauté the onion and garlic until translucent, about 3 minutes.
Add the mustard greens to the skillet, cooking until wilted, roughly 5 minutes. Tip: A splash of water helps steam the greens faster.
In a large bowl, whisk together the cornmeal, flour, baking powder, salt, and sugar.
In another bowl, mix the milk, vegetable oil, and egg until well combined.
Pour the wet ingredients into the dry ingredients, stirring just until combined. Tip: Overmixing can make the cornbread tough.
Spread the sautéed greens evenly in the prepared baking dish, then sprinkle with cheddar cheese.
Pour the cornbread batter over the greens and cheese, spreading it gently to cover.
Bake for 25 minutes, or until the top is golden and a toothpick inserted comes out clean. Tip: Let it sit for 5 minutes before serving for easier slicing.
