In a saucepan saute the salt pork and onions until onions are slightly translucent, and salt pork is cooked. Add the rest of the veggies, and clams with juice, and clam juice. You may need more clam juice, just enough to cover veggies. Bring to a boil.
In the meantime, in another saucepan, melt the butter with the flour and stir together. In a small cup, stir the cornstarch together with 3 tbsp of cold water. Add the half and half to the butter mixture, then add the cornstarch mixture , stir in the milk. Bring to a simmer to thicken stirring well.
When veggies and potatoes are done , add to the white sauce mixture, and heat through.