Ingredients

• 1.2 kg potatoeswaxy or all-purpose, so they slice cleanly
• 500 g boneless chicken thighsthighs stay juicier than breast over the longer bake
• 180 g thick-cut smoked streaky bacon
• 2 leekssliced
• 2 garlic cloves
• 400 ml double cream
• 150 ml whole milk
• 120 g mature cheddar
• 50 g Parmesan
• 1 tsp Dijon
• 1 1/2 tsp fine salt, plus black pepper and a grating of nutmeg

Method

1. 01

   Render the bacon

   Put the bacon into a cold, dry frying pan over medium-low heat and cook for 6-8 minutes until crisp, so the fat renders out slowly. Lift it out with a slotted spoon and leave the fat behind.

   180 g thick-cut smoked streaky bacon

2. 02

   Brown the chicken, soften the leeks

   Raise the heat to medium-high and brown the 500 g diced chicken in the bacon fat in two batches, 3-4 minutes each. Remove, then add the sliced leeks and cook over medium heat for 6-8 minutes until soft and sweet, scraping up anything stuck to the base.

   180 g thick-cut smoked streaky bacon · 2 leeks

3. 03

   Infuse the cream

   Warm the cream and milk in a saucepan with the crushed garlic cloves, Dijon, the salt, pepper and a fine grating of nutmeg until steaming but not boiling, then take it off the heat and leave it for 10 minutes. Infusing now means every layer of the gratin is seasoned, not just the top.

   400 ml double cream · 150 ml whole milk · 2 garlic cloves · 1 tsp Dijon

   Resting is part of cookingFifteen minutes lets the cream set enough to serve in squares rather than spoonfuls.

4. 04

   Layer

   Butter a deep 23 x 33 cm dish and build it up in layers: potatoes, then chicken, leeks and bacon, seasoning very lightly as you go, finishing with a neat layer of overlapping potato slices on top.

   Slice the potatoes evenlyTwo to three millimetres. Uneven slices leave some layers firm when the top is brown.

5. 05

   Bake covered

   Pour the infused cream evenly over, pressing the top layer down so it is submerged. Cover with foil and bake at 180 C / 160 C fan for 45 minutes.

6. 06

   Uncover and finish

   Remove the foil, scatter over the cheddar and Parmesan, and bake for a further 25-30 minutes until bubbling, blistered and deeply browned. A knife should slide through the centre with no resistance. If it catches, give it another 10 minutes covered.

   120 g mature cheddar · 50 g Parmesan

   Test with a knife, not the clockOvens and potatoes vary. No resistance at all in the centre before it comes out.

7. 07

   Rest

   Rest for 15 minutes before serving. A gratin straight from the oven runs across the plate; rested, it holds a clean edge.