Rinse peppers under cold water and pat dry. Slice off tops and bottoms, stand upright, and cut down along the inside to remove seeds and white ribs. Slice lengthwise into ½-inch strips; halve crosswise for shorter pieces. Keep sizes uniform so everything cooks evenly. If adding onion, slice it now. Measure out olive oil, salt, and pepper. Have everything ready before the pan goes on — sautéing moves fast.

Heat a large Skillet over medium heat. Add 1 tbsp olive oil per 2 peppers; warm 30–60 seconds until shimmering. Add pepper strips in a single layer — work in batches if crowded. Season with salt and pepper, stir to coat. Cook over medium heat, stirring every 2–3 minutes, for 6–10 minutes. At 6 minutes: slightly softened with snap. At 10 minutes: tender throughout with light color. For very soft peppers, cook 30–40 minutes over medium-low, stirring occasionally. Add garlic halfway through cooking. If browning too fast, add a splash of white wine or water and briefly cover to steam.

Gradually pour in the 2 ½ cups whole milk warmed, a splash at a time, whisking vigorously after each addition to prevent lumps. Once all the 2 ½ cups whole milk warmed is incorporated, reduce the heat to medium-low and continue stirring for 8–10 minutes until the béchamel is very thick — it should pull away from the sides of the pan and hold its shape on a spoon. Stir in the diced 5 oz jamón serrano finely diced, ¼ tsp freshly grated nutmeg, ¾ tsp fine sea salt plus more, and 1 pinch white pepper. Taste and adjust seasoning.

Spread the béchamel mixture evenly onto a lightly greased Baking Sheet or into a shallow dish. Press a sheet of Plastic Wrap directly onto the surface to prevent a skin from forming. Refrigerate for at least 2 hours, or overnight, until completely firm and cold.

Rinse peppers under cold water and pat dry. Slice off tops and bottoms, stand upright, and cut down along the inside to remove seeds and white ribs. Slice lengthwise into ½-inch strips; halve crosswise for shorter pieces. Keep sizes uniform so everything cooks evenly. If adding onion, slice it now. Measure out olive oil, salt, and pepper. Have everything ready before the pan goes on — sautéing moves fast.

Heat a large Skillet over medium heat. Add 1 tbsp olive oil per 2 peppers; warm 30–60 seconds until shimmering. Add pepper strips in a single layer — work in batches if crowded. Season with salt and pepper, stir to coat. Cook over medium heat, stirring every 2–3 minutes, for 6–10 minutes. At 6 minutes: slightly softened with snap. At 10 minutes: tender throughout with light color. For very soft peppers, cook 30–40 minutes over medium-low, stirring occasionally. Add garlic halfway through cooking. If browning too fast, add a splash of white wine or water and briefly cover to steam.