The onion compote is ready to use as soon as it is made, or it can be stored in a covered container in the fridge for up to 3 days. Heat a well-seasoned sauté or frying pan with a lid over a medium-high heat, then add the butter with a splash of oil. When the butter is foaming, add the onions, garlic and thyme and season with salt and pepper. Cover with a wet piece of greaseproof paper, cover the pan with the lid, turn the heat to very low and leave the onions to sweat for 20–25 minutes until they are very tender. Transfer to a plate and set aside to cool.
When you’re ready to cook, preheat the grill to high.
Meanwhile, place a partridge breast in between 2 sheets of clingfilm and use a meat mallet or rolling pin to lightly bash until flattened. Repeat with the remaining breasts. Pat the breasts dry and season them all with salt and pepper.
Heat a well-seasoned sauté or frying pan over a medium-high heat, then add a splash of oil. When it’s hot, add the breasts and sear them for 2–3 minutes on each side until cooked through, then set aside to rest for 5 minutes covered with kitchen foil. Fry the breasts in batches, if necessary.
Toast the sourdough slices on both sides under the grill, then lightly butter each. Do not turn off the grill.
Spread the pieces of toast with the onion compote and add the egg slices. Top with the partridge breasts, crispy pancetta, watercress and finally the cheese. Place the toasties under the grill until the cheese melts, then lightly season with salt and cracked pepper.