Trim both the ends of green beans. If can also cut the beans in half instead of keeping it full.
Heat butter and oil in an oven-safe skillet (preferably cast-iron skillet).
Add green beans to the hot skillet. Season it with salt and pepper. Mix well.
Cover and cook on medium heat for 5-6 minutes on medium-high heat. Toss once in between.
Remove the lid. Add minced garlic and toss well. Cook for another 30 seconds until the garlic has browned.
Switch off the flame. Sprinkle parmesan cheese and mozzarella cheese. Broil @200 C for 2-3 minutes until the cheese start to bubble.
Notes
Use a cast-iron skillet for a nice charred effect on green beans.
The idea is to sear the green beans on medium-high heat to get it charred. And covering the skillet with a lid briefly creates steam and cooks the beans just right, keeping it tender.
Add garlic towards the end to prevent it from burning.
Keep an eye on salt, as the cheese also has salt in it.
Sometimes I add a splash of heavy cream before adding cheese, it tastes very close to traditional green beans casserole.