Preheat the oven to 400F.
Heat 1 tablespoon butter in a large skillet over medium heat. Add the onions and cook for 4 minutes, until translucent. Add the celery, carrots, and thyme and cook an additional 3 minutes, until the veggies are soft.
Add the remaining 3 tablespoons butter to the veggies, sprinkle with the flour and stir to make a roux, cook for 1-2 minutes.
Add the milk, vegetable broth, potatoes, peas, corn, and salt and stir to combine.
Reduce the heat to low and simmer 10 minutes, until the sauce is thick and creamy and the potatoes are soft.
Divide the filling between ramekins and top with pie crust, gently pressing the dough onto the edges of the ramekins. Use a knife to make 3 slits in the top of the dough for ventilation.
Place the ramekins on a baking sheet in the oven and bake for 20-30 minutes, until the crust is golden brown.