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1
Heat oven to 400 degrees.
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2
Put oil in a large skillet over medium heat.
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3
When oil is hot, add the leek, sprinkle with salt and pepper, and cook, stirring occasionally, until softened, about 5 minutes.
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4
Add mushrooms and cook until liquid has released and evaporated, about 8 to 10 minutes.
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5
Add stock and rosemary; bring to a boil, and let bubble for a minute or two, then add carrots and chicken and reduce heat so the liquid simmers.
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6
Cook until carrots are almost tender and chicken is cooked through, 8 to 10 minutes.
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7
Add peas and cook, stirring occasionally, until vegetables are brightly colored and just tender, another minute or so; fish out rosemary stem and discard.
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8
Whisk cornstarch with a few tablespoons of broth to make a slurry.
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9
Add slurry to pot and stir until liquid thickens slightly.
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10
Transfer everything to an ovenproof dish and set aside.
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11
Put flour in a food processor with baking powder, soda and salt.
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12
Add butter and process until mixture resembles small peas, no more than 30 seconds.
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13
(You can also do this by hand, using two knives, a fork, your fingers, or a pastry cutter).
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14
Transfer mixture to a bowl and mix in buttermilk and egg until it just comes together; it should be sticky.
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15
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.
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16
Bake for 35 to 45 minutes until golden on top and bubbly underneath.
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17
Scoop into bowls and serve immediately.
