Preheat to 400 F. Brush a large baking tray with 2 tablespoons olive oil.
Spread the green beans, onions, and garlic cloves on the tray and sprinkle lightly with salt and pepper.
Bake for 20 minutes, intermittently stirring or shaking the tray. Taste a green bean to see if it's as tender as you like it. If not, put it back in for another 5 to 10 minutes.
Remove from oven; transfer to a bowl. Drizzle with vinegar, and possibly grind in some additional black pepper. Serve at any temperature, topped with lightly toasted pine nuts.
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