Place the oil in a large skillet set over medium heat. Add the onion, carrot and celery and cook until tender, about 5 minutes. Mix in the garlic and flour and cook 2 minutes more. While stirring, slowly mix in the stock. Bring the mixture to a simmer and simmer until the vegetables are tender, about 8 minutes. Stir in the salmon and simmer until just cooked through, about 5 minutes. Mix in the peas, corn, dill, salt and pepper and simmer until the frozen vegetables are heated through. If too thick, thin the stew with a bit more stock.