Leave green beans sitting out while for about 30-60 minutes or defrost in microwave for 3-4 minutes
You want the green beans to be just defrosted but not warm or soggy
In a small skillet over medium high heat melt butter along with olive oil
When butter is melted and pan is warmed enough that the green beans will sizzle then add all the green beans
Continue sauteing and stir occasionally for about 3-4 minutes
Add garlic and onion and continuing stirring for another 3-4 minutes until green beans are heated through and garlic and onion is crispy and starting to brown (but not burned!)