1. Preheat a combination oven to 145 °C.
2. Arrange the onion and rosemary in a roasting tray .
3. Place in the middle the pork loin rubbed with salt and pepper.
4. Roast pork until the core temperature is 40 °C.
5. Continue cooking with dry heat about 110 °C, until the core temperature is 68 °C.
6. Cool in the blast chiller.
7. Transfer pan drippings to a saucepan. Bring to a boil and cook until it's reduced by half.
8. Blend the reduced pan drippings, add chestnuts and cook few minutes.
9. Make the roux with the melted butter and the flour.
10. Pour milk in the pan drippings and whisk to combine with the roux, until thickened to a gravy consistency.
11. Slice the chilled pork loin (not too thin).
12. Reheat the pork with the combination oven set at 135 °C, about 4 minutes.
13. Plate up, spooning hot sauce over the pork loin.