01 - Preheat the oven to 375°F. Lightly grease a 12-cup muffin tin with cooking spray or butter.
02 - Bring a pot of salted water to a boil. Add green beans and blanch for 3 minutes. Drain and immediately plunge into ice water to stop cooking. Set aside.
03 - In a skillet over medium heat, melt butter with olive oil. Add onions and cook for 3–4 minutes until translucent. Add garlic and cook for 1 minute until fragrant.
04 - Sprinkle flour into the skillet, stirring constantly for 1 minute. Gradually whisk in milk and cook until slightly thickened, about 2 minutes.
05 - Remove skillet from heat. Stir in sour cream, 3/4 cup Parmesan, salt, and pepper until smooth. Add green beans and mix to coat evenly.
06 - Divide the green bean mixture among the muffin cups, pressing down gently to pack each cup.
07 - In a small bowl, combine panko breadcrumbs with remaining 1/4 cup Parmesan. Sprinkle evenly over each filled cup.
08 - Bake for 18–20 minutes, or until tops are golden brown and filling is bubbling.
09 - Let cool for 5 minutes before carefully removing cups from the tin. Serve warm.