heat oven to 400 degrees. heat oil in a large pot over medium heat. add a sliced leek, with salt and pepper, and cook 5 minutes. add two cups quartered button mushrooms and cook 10 minutes. add 1½ cups chicken stock and some rosemary in a tea ball. bring to a boil for a few minutes, then add three chopped chicken thighs and two chopped carrots, and reduce heat to a simmer, and cook for ten minutes. add 1 cup peas and cook until vegetables are tender.
whisk cornstarch with a few tablespoons of broth to make a slurry. add slurry to pot and stir until liquid thickens slightly. transfer everything to an ovenproof dish and set aside.
put 1 cup flour in a food processor with ¾ tsp baking powder, ¼ tsp soda and ¼ tsp salt salt. add 3 tbsps chopped butter and process 30 seconds. transfer mixture to a bowl and mix in ½ cup buttermilk and 1 egg until it just comes together; it should be sticky.
drop spoonfuls of batter on top of vegetables and chicken and smooth with a knife, covering as much surface area as possible but leaving a few gaps for steam to escape. bake for 35 to 45 minutes until golden on top and bubbly underneath. scoop into bowls and serve immediately.
