Melt butter in a large pot. Add chopped onion, matchsticks carrots, and celery. Stir to combine. Saute for 5-7 minutes or until veggies are soft. Add minced garlic and saute for 1-2 minutes. Then, add all-purpose flour. Mix to combine and cook for 1 minute more. Pour evaporated milk and chicken stock into the veggies mixture. Mix and bring to a boil.
Add shredded chicken,dried thyme, pepper, and salt. Simmer for 5 minutes.
Prepare the dumplings while the chicken soup is simmering. Mix together all-purpose flour, baking powder, pepper, salt, and thyme in a large bowl. Then, add melted butter and whole milk to these dry ingredients. Mix using a rubber spatula until combined, and the mixture looks like dough.
Form the dumplings using an ice-cream scoop. Add dumplings to the chicken soup. Place the dumplings with some distance between them, because they will increase in size. Lightly dip the dumplings into the soup. Cover with a lid and simmer for 15 minutes or until the dumplings are cooked (gently stir the chicken soup with dumplings a few times). Check the readiness of the dumplings by halving one of them with a spoon. Serve warm.