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In a large frying pan, heat the oil over a medium heat and cook the onion for 3-4 minutes. Add the mushrooms and cook for another 4-5 minutes, until softened. Stir in the kale and stir-fry for 1 minute.
Remove the pan from the heat and add the chestnuts, mashing in the pan using a fork.
Sprinkle over the crumbled feta, oregano and mix together well. Season to taste.
Slice the tops of the peppers and set aside. Deseed the peppers, then divide the mushroom and chestnut mixture equally between them, pressing down into the cavity.
Stand the peppers in the dish of a slow cooker and replace the tops. Set the slow cooker to low and cook for 5 to 5½ hours, until the peppers are soft.
Meanwhile, make the pesto: in the small bowl of a food processor, blitz the watercress with the nuts, until they are broken down. Add the olive oil and lemon juice and season, then process again until nearly smooth.
Add a little water depending if you prefer a looser consistency. Serve the peppers with a drizzle of pesto.