Chicken potpies are as American as apple pie, but they needn't be as fattening. Insteadofa buttery piecrust, cap them with a billowy mound of light mashed potatoes, whichsoaks up the flavorful broth in every spoonful.
Make the potatoes: Place potatoes and garlic in a medium saucepan. Cover with cold water; bring to a boil over high heat. Reduce heat to a bare simmer; cook until potatoes are easily pierced with a fork, about 15 minutes. Drain; pass through a food mill into a large bowl. In the same pan, heat milk, butter, and the salt. Pour over potatoes; stir to combine. Season with pepper; cover, and keep warm.
Make the filling: Heat broiler with rack in lowest position. In a medium saute pan, heat the oil over medium heat, and add shallot. Cook until soft, about 3 minutes. Stir in the mushrooms and the salt. Cook, stirring occasionally, until lightly golden and beginning to soften, about 8 minutes. Add parsley and thyme; remove from heat. Divide evenly among four 2-cup ovenproof dishes.
Bring stock, 2 cups water, and bay leaf to a boil in a medium saucepan; add leeks and chicken. Reduce heat to medium; simmer until chicken is cooked through, about 4 minutes. Add peas; immediately turn off heat. Using a slotted spoon, divide chicken mixture evenly among baking dishes. Reserve 1 cup cooking liquid in saucepan; set aside remainder for another use (store in an airtight container in refrigerator up to 2 days).
In a small bowl, combine cornstarch with about 1 tablespoon water; whisk until smooth. Slowly whisk into reserved broth in saucepan, and bring to a simmer over medium heat. Cook 1 minute, and ladle into baking dishes, dividing evenly. Spoon about 1 cup potato mixture on top of each dish, and place under broiler until tops begin to brown, 10 to 12 minutes. Remove from broiler, and serve hot.