In a large pot over medium-high heat, sauté onion and garlic in olive oil. Cook until translucent and fragrant. Add stock and the next 7 ingredients (through kosher salt). Boil until the vegetables are fork tender, about 15 minutes.
Stir in baby kale. Make a slurry by whisking together the flour and soymilk. Stir in and simmer until semi-thickened. Pour the potpie filling into a 10-inch cast iron skillet, place puff pastry on top. Bake for 30 minutes, or until the top is golden brown.