Bring a large pot of water to a rolling boil. Add fresh green beans and blanch them for exactly 1 minute until they turn bright green and are slightly tender. Quickly transfer the beans into a bowl of ice water to stop the cooking process. Let them cool for a few minutes, drain well, and set aside.

In a large skillet, melt 1 tablespoon of butter over medium-high heat. Add sliced onions, seasoning them with a pinch of salt and pepper. Sauté for about 5 to 8 minutes until the onions are soft and translucent. Lower the heat and continue cooking for an additional 15 minutes, stirring occasionally, until they turn a deep golden brown and caramelized.

If the skillet begins to dry out, add 1/4 cup of chicken or vegetable broth to deglaze, scraping up any browned bits from the bottom. Stir until the liquid reduces and the onions are well-coated in flavor. Add minced garlic and cook for about 1 minute, stirring frequently until fragrant.

Wipe any excess fat from the skillet and return it to medium heat. Add another tablespoon of butter and let it melt before incorporating the blanched green beans. Season with red pepper flakes (if desired) and a pinch of salt and pepper. Sauté for 1 to 2 minutes until heated through.

Top the green beans with the caramelized onions from the skillet. Gently toss to combine and serve hot as an irresistible side dish.