Make filling: Place the chicken breasts in a large saucepan and fill with enough cool water to cover them by 1 inch. Bring to a boil over medium-high heat, then reduce the heat to low, cover, and let simmer for about 15 minutes (check them after 10), until the chicken is completely opaque and cooked through. The thickest part on the breast should register 165 degrees F on a meat thermometer. Drain and transfer the chicken to a cutting board and let cool before chopping into 1-inch chunks.

Preheat the oven to 350°F. Spray an 8-inch baking dish (or any casserole dish of a similar size) generously with cooking spray.

In a large saucepan or dutch oven, heat the butter over medium heat. Add the onion, carrots, celery, and corn, and cook for about until tender, about 10 minutes. Add the garlic and stir constantly for 30 seconds. Sprinkle the flour into the pan and stir until it coats the vegetables, about 30 seconds. Slowly stir in the chicken stock, stirring well so that the flour begins to de-clump and thicken the stock. Cook the mixture over medium heat until thickened and bubbly, about 4 minutes. Stir in cooked chicken, thyme, sage, salt, and pepper. Pour the mixture into your prepared dish and spread evenly.

Make the crust: In a large bowl, whisk the cornmeal, flour, baking powder, sugar, and salt. In a separate bowl, whisk the milk, egg, and canola oil until smooth. Stir the wet ingredients into the dry ingredients. Spoon the batter evenly over the filling, smoothing the top. Bake until the top is puffed and golden brown, 22 to 25 minutes. Let cool for at 10 minutes before serving.