Corta los extremos del tallo; deja las puntas ahusadas. Enjuaga, luego seca bien los ejotes — la humedad en la superficie causa cocción al vapor en lugar de asado. Mezcla en un tazón grande con 1 a 1½ cucharadas de aceite de oliva por libra. Sazona generosamente con sal y pimienta negra. Opcional: ajo en polvo, ajo picado, o chalotes rebanados. Guarda cualquier Parmesano para los últimos minutos de asado.

Preheat oven to 425°F — fully up to temp before beans go in. Line a large rimmed sheet pan with foil or parchment. Spread seasoned beans in a single layer; no overlapping or crowding or they'll steam instead of roast. Large batches: use two pans, one on upper rack, one on lower. Roast at 425°F for 14–20 minutes. Thin beans (haricots verts): 14–15 min. Thicker beans: 18–20 min. Halfway through (7–10 min), shake the pan or use tongs to turn the beans. Swap rack positions if using two pans. Done when tender at center with lightly blistered, browned edges. Adding Parmesan? Sprinkle it on in the last 3–5 minutes and return to oven until melted and lightly crisp.

Done when tender but still holding shape, with browning or light char on edges and tips — color shifts from bright to deeper olive-green. Serve immediately; they soften as they sit. Key mistakes: overcrowding the pan (causes steaming, not roasting) and skipping the drying step after washing. Prep and toss in oil ahead if needed, roast just before serving. Leftovers keep refrigerated up to five days; reheat in a low oven.