Saute chicken in 1 TBSP oil until almost cooked through. Set aside.
Heat remaining TBSP of oil in a large saute pan or dutch oven. Add onion, carrots, celery and sage. Cook on medium-high heat for about 8-10 minutes, until the vegetables are soft, stirring often.
Add the cans of chicken soup, broth and milk to the vegetables. Whisk to combine. Ladle out 1/4 cup of the liquid and mix in the cornstarch. Return cornstarch liquid to pan. (If you just add the cornstarch to the larger mixture, it's really hard to incorporate. So mix it into a small amount, then pour that into the stew.)
Add the chicken and the frozen peas. Heat until bubbly, then reduce to a simmer, cover and cook for 20 minutes.
While the stew is simmering, you can mix up the dumplings. Add the dry ingredients into a bowl and whisk thoroughly. Add the butter and milk and stir, using a fork or a rubber scraper until well combined.
After 20 minutes, remove the lid and increase the temperature back to medium heat. Give the stew a quick stir, then add the dumplings to the stew in 1 TBSP sized balls. (You'll have about 20 dumplings.) The dumplings will rise & expand while cooking so make sure you're saute pan is large enough to accommodate.
Replace the lid and turn the heat down to low. Cook the mixture for another 20 minutes, with the lid on, without stirring or ever lifting the lid!
Gently stir and serve. Enjoy!