For the Casserole: Grease a 9x13 glass baking dish to hold the casserole and preheat the oven to 350 degrees F.
Add 1 tablespoon olive oil to a large saute pan or Dutch oven and place on the stovetop over medium heat. Add the chicken and sprinkle with 1 tablespoon Herbes de Provence, 1/2 teaspoon kosher salt and half of the minced garlic.Cook until the chicken is lightly browned all over and has released any retained water, about 3-4 minutes. It doesn't need to be cooked all the way, as it will be baked again. Using a slotted spoon, transfer the chicken to the casserole dish. Remove any liquid from the pan and set aside.
Add the remaining olive oil to the pan along with the remaining garlic, Herbes de Provence, salt, leeks, carrots and celery. Cook over medium heat about 5 minutes until fragrant and the vegetables are slightly soft. Add the frozen peas and stir to combine. Transfer the vegetables to the casserole dish and gently combine with the chicken.
In a medium bowl, combine the cream of celery soup and oat milk. Whisk to combine and pour the mixture over the casserole. The liquid should moisten all the contents of the casserole, coming about half way up the sides of the dish. If more liquid is needed, add some of the reserved cooking liquid from the chicken.
Put the casserole in the oven.For the Dumplings:
As the casserole begins to bake, make the dumplings. In a medium bowl, whisk together the dry ingredients.
Remove the butter from the refrigerator and grate the cold butter over the dry ingredients. Stir to combine. The mixture will be a little pebbly and thats OK.Make a well in the center of the ingredients and pour in the yogurt or buttermilk. Stir gently and quickly to combine until the dough forms. Gently squeeze and pat the dough into a ball that comes together but don't remove it from the bowl.
Take the casserole out of the oven and increase the oven temperature to 400 degrees.
Pinch off a piece of dough and gently form a ball in the palm of your hand, about 1-1/2" across. Using a soup spoon, create a cavity in the hot casserole and nestle the ball of dough inside. Repeat until all the dough is used up, studding the casserole with about 8-9 dumplings. Brush the top of each dumpling with a little bit of oil or melted butter.
Return the casserole to the oven and bake 15-20 minutes more until bubbling hot and the dumplings are flaky inside.
Finish with freshly ground pepper and salt to taste.
Notes:
Super kudos to Nicole Hunn for the perfect, easy Gluten Free Drop Biscuit recipe, included as a half batch for this casserole. It was the perfect amount and perfect texture!
