Preheat the Oven: Set your oven to 425°F (220°C). This high heat will help the green beans roast perfectly.

Caramelize the Onions: In a large non-stick skillet, heat 1 tablespoon of olive oil over medium-high heat. Add the sliced onions, season with a pinch of salt and black pepper, and cook, stirring occasionally, until the onions start to brown (about 10 minutes). Lower the heat to medium-low and cook for another 30 minutes until the onions are deeply golden and caramelized, stirring frequently.

Prepare the Green Beans: While the onions are caramelizing, spread the trimmed green beans on a baking sheet. Lightly spritz them with olive oil spray and season with salt and black pepper.

Roast the Green Beans: Place the baking sheet with the green beans into the preheated oven. Roast for 16-18 minutes, flipping the green beans halfway through for even cooking. They should be tender-crisp with a slight char on the edges.

Combine and Serve: Once both the green beans and caramelized onions are ready, transfer the green beans to a platter and top them with the caramelized onions. Serve hot as a side dish.